Mutagen

Mutagen, also known as Retromutagen Ooze, is a mucilaginous 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 rapidly increase in size, strength and intelligence. It is responsible for mutating, among others, Splinter and the four Ninja Turtles.

1987 cartoon
The mutagen was depicted slightly differently in this cartoon; rather than causing its victims to increase in size, it caused them to transform into a hybrid of their original species and whatever creature they have most recently been in contact with. It was revealed in the episode Invasion of the Punk Frogs that the mutagen can be made mixing nyotrinaline with Taxon-9. It is also responsible for creating many of the show's mutants, such as Bebop, Rocksteady, Mona Lisa, and Mondo Gecko, to name but a few.

Movies
As opposed to being labeled T.C.R.I. (Techno Cosmic Research Insitute) in the Mirage comics, the canister in which the ooze is kept is written as "T.G.R.I." (Techno Global Research Institute), in compliance with the plot. In the first film, the mutagen is shown as having been the substance that mutated Donatello, Raphael, Michaelangelo, Leonardo, and Splinter from ordinary animals into anthropomorphic beings.

Mutagen was a much more integral part of the second movie, Teenage Mutant Ninja Turtles II: The Secret of the Ooze. The Foot Clan had abducted Professor Jordan Perry and forced him to create a new batch of ooze. This mutagen was used to mutate a wolf cub and a baby snapping turtle into Rahzar and Tokka, respectively. Near the end of the film, Shredder drank some mutagen to become Super Shredder.

Jordan Perry explains to the Turtles that the ooze that mutated the Turtles and Splinter was a mistake. In Perry's words, "an unknown mixture of discarded chemicals was accidentally exposed to a series of radiated waves", results in the creation of the ooze which has "remarkable, but dangerous mutagenic properties." Donatello was pretty much disappointed and upset upon learning this, he always hoped that he found out the Turtles and Splinter were special, and believes that there has got to be more to it.

The canister was seen placed among the helmet of Shredder, the Time Scepter, and Yaotl's helmet in his trophy room in TMNT.

In Turtles Forever, there were small capsules of what appeared to be purple mutagen which Donatello took possession of from the 1988 Shredder. At least two members of the Purple Dragons were mutated by the mutagen. Some unnamed henchman was mutated into a half-dog mutant while Hun was mutated into a half-turtle which was similar to Slash. A great deal of mutagen was mass produced by Ch'rell and Karai. They used it to mutate human foot soldiers into a mutant army which they used along with upgraded robot 1988 Foot Soldiers in the foot's attack on New York in the 2003 universe.

Video Games
In the first Teenage Mutant Ninja Turtles GBA game, Mutagen canisters are used as health items.