Mutagen, also known as Mutagenic Ooze, is a mucilaginous, radioactive, neon green, glowing chemical substance that has served as a key plot device in nearly every medium of the Teenage Mutant Ninja Turtles. Any creature that is exposed to mutagen will be affected in a myriad of ways, from growing in size and intellectual capacity, merging with inorganic materials such as metal, or gaining superpowers such as telepathy or telekinesis. It is responsible for mutating, among others, Splinter, the four Turtles, and villains such as Rahzar and Fishface.
In this series, the Mutagen is depicted as having several similar properties to the version seen in the 1987 series; It blends with the victim's genetics with that of whatever creature they have last made contact with. It has a glowingly blue/green coloration to it.
Cockroach Terminator - The Mutagen also has the ability to merge organic and inorganic substances, as seen with Spy-Roach. The same subject shows that it may also cause similar effects to the original comic series' mutagen, both increasing size and intelligence.
The Pulverizer Returns! - The Kraang operative told Shredder that the effects of the Mutagen on Earth are unpredictable. If a human has not come into contact with another living being before being effected by Mutagen, their bodies will be molecularly broken down, turning them into a mutant blob made of acidic goo, with only their internal organs still remaining intact. Pulverizer also states that the Mutagen burns upon contact.
Mikey Gets Shellacne - A mutant's exposure to a secondary dose of mutagen can accelerate the original mutation; when Bradford (as Dogpound) falls into a vat of mutagen, he is further mutated into a new form Mikey dubbed as Rahzar.
The Kraang Conspiracy - It has been stated that the mutagen was not intentionally created as a mutagenic agent, and apparently had a different effect back in Dimension X, but the new laws of physics in our dimension altered its function.
The Lonely Mutation Of Baxter Stockman - It is revealed that April is immune to mutagen.
Into Dimension X! - In this episode, the Kraang perfected the mutagen, revealing it's true purpose; to "terraform" Earth into a new Dimension X. Objects such as rocks or trees sprayed with the perfected mutagen transform into Kraang elements; creatures including humans are mutated into Kraang monsters.
City at War - Stockman-Fly started feeding mutagen to Shredder while he was recuperating.
The Insecta Trifecta - The mutagen that was injected into Shredder started to take its effect, causing him to sprout razor-sharp blades from the backs of his hands.
The contact with mutagen causes one to merge with the DNA of the last creature one came in contact with. If one has not recently came in contact with any living being it causes one to mutate into an undefined blob, as seen with Timothy.
It is shown that when exposed to inorganic matter such as cement, it causes it to dissolve like an acid. Considering Timothy stated that mutagen burned him, mutagen in its base form might just be an acid.
If one is physically hurt when mutated, the wound will reflect on its final form. For example, Chris Bradford was bitten by Shredder's dog on his right hand. As a result Bradford's wounded arm did not grow properly when he transformed into Dogpound.
If ingested the mutagen causes one to mutate into the last thing ingested before the mutagen. For example, Antonio mutated into Pizza Face as pizza was the last thing he ate; a nameless cat mutated into Ice Cream Kitty, as she ingested a mix of mutagen and Ice cream. The only known exception are the Squirrelanoids, whose consumption of mutagen simply made them monstrous.
Oddly, the mutagen causes some subjects to lose their intellect in some rare cases. For example, Dr. Tyler Rockwell and Dr. Kirby O'Neil who became the instinctive beasts "Monkey Brains" and "Kirby Bat", respectively.
In some rare occasions when an animal hasn't recently come in contact with a human, the exposure to mutagen causes is to develop a human level intellect as well as an anthropomorphic mutation. One example would be the The Punk Frogs.
The effects of fusion are similar to the regular exposure. The main difference is that, as the name implies, fusion merges two or more living creatures instead of just mutating one based on others DNA. For Example, Mega Shredder and Creepweed.
- It can also cause the merging of organic with inorganic matter. For example, "Cockroach Terminator", which is the fusion of a cockroach and spy equipment.
Secondary mutations are known to cause, as referred by Mikey, "super mutants". It occurs when a mutant is exposed to mutagen for a second time. One example of secondary mutation is Rahzar, an evolved version of Chris Bradford's first mutant form. In Raphael: Mutant Apocalypse, an alternate Leonardo was also shown to be double-mutated, and suffered from amnesia.
Monkey Brains - Dr. Victor Falco produced a specific version of the mutagen that would not only mutate Dr. Rockwell but also give him psychic powers. Later Falco perfected the serum, giving him the ability to read minds.
I, Monster - While using rats to test his psychic serum, Falco suffered an accident that gave him new abilities.
Parasitica - Donatello stated that the mutagen effects are usually anthropomorphic, which is not the case of the parasitic wasp, which merely increased in size.
The Lonely Mutation of Baxter Stockman - Donnie develops a cure for Kirby and Splinter's mutations, this time called Retro-Mutagen. It requires about ten canisters of mutagen, April's DNA, and several months to create one small dose of antidote.
Vengeance is Mine - Stockman accidentally modifies the mutagen that mutated Karai, making it possible for her to shift back into her human form. As was revealed later in The Pig and The Rhino, this unique mutation can not be reversed by using regular Retro-Mutagen.
Within the Woods - Donatello created a mutagen-based medicine for Leo. "Leo's Elixir" did not mutate him, but once it was out of his body it caused the Creep's mutation. The Creep worships the mutagen and had an altar for Leo's Elixir.
Race with the Demon - Speed Demon spills some Mutagen-infused gasoline on the road, which later causes the mutation of Dr. Cluckingsworth M.D., who's able to lay Radioactive Mutagen Eggs. Donnie and Casey use the eggs as fuel for the Turtle Racer.
Battle for New York, Part 2 - Michelangelo accidentally created a super retro-mutagen formula that can transform huge amounts of mutagen into retro-mutagen. The Turtles stole a Technodrome and used its mutagen tank to de-mutate the entire population of New York, including Kirby.
In the beginning of second season Donatello developed devices that would help him and his brothers to look for the missing mutagen canisters.
- This version of mutagen has a far more varied array of resulting mutations than other variants.
- Good or neutral mutants are fairly humanoid, while amoral characters exposed to the ooze become monstrosities. This is best demonstrated by Xever Montes: both he and Bradford mutated, but the pragmatic Bradford remained fairly humanoid as Dogpound, while the sociopathic Montes became a fish with arms as Fishface. And after Dogpound mutated into Rahzar next season, he was even more sadistic.
- Mutations are generally anthropomorphic and animalistic, usually someone mutates into the last anthropomorphic and animalistic object/animal they come into contact with, albeit rather hideously deformed/disfigured.
- Hamato Yoshi/Splinter was mutated into a large anthropomorphic brown sewer rat because he stepped on a New York sewer rat's tail, while the Turtles became anthropomorphic, taking on human-like abilities and characteristics because they were in Yoshi's hands.
- Baxter Stockman was mutated because a housefly landed on his nose.
- The Season 2 finale shows distinct differences between a "pure" mutation and a typical one - particularly with the humans, who in the finale turned into Kraang-like monsters rather than the usual, animal-based mutants. This was because the Kraang perfected the Mutagen.