Mutagen (Golden Harvest/Imagi)

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 (sometimes), and even gain superpowers. It is responsible for mutating, among others, Splinter, the four turtles, and villains such as Tokka & Rahzar.

Teenage Mutant Ninja Turtles (1990 film)
As opposed to being labeled T.C.R.I. (Techno Cosmic Research Institute) in the Mirage comics, the canister in which the ooze is kept is written as "T.G.R.I." (Techno Global Research Industries), 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.

Teenage Mutant Ninja Turtles II: The Secret of the Ooze
Mutagen was a much more integral part of the second movie. Having identified TGRI as the source of the Ooze, the Foot Clan abducted Professor Jordan Perry and forced him to create a new batch of ooze, to create new warriors against the turtles. This mutagen was used to mutate a wolf cub and a baby snapping turtle into Rahzar and Tokka, respectively (Although Professor Perry was able to modify the new ooze to retard the subjects' intellectual capacities, both creatures being relatively infant-like where Splinter achieved human-level intellect fairly quickly and the turtles were only limited by their initial youth).

Near the end of the film, Shredder drank some mutagen to become Super Shredder. It is unknown what, besides his natural human form, animal species' attributes the Shredder gained during this time. 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 very disappointed and upset upon learning this, as he always hoped that there would be more to their origins than an accident, but Splinter encouraged him not to judge his worth based on his origins.

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

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. The first was Mohawk, who 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 1987 Foot Soldiers in the foot's attack on New York in the 2003 universe.

Anti-mutagen
In Teenage Mutant Ninja Turtles II: The Secret of the Ooze, it was mentioned that the neutralizer is made of dimethyl chloride and must be heated to 338 Kelvin (65° Celsius or 149° Fahrenheit), which must be consumed in order to be effective; the turtles were able to trick Tokka and Rahzar into taking the anti-mutagen by freezing it into ice cubes and hiding the cubes in doughnuts.