Mutagen, also known as 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). It is responsible for mutating, among others, Splinter, the four turtles, and villains Tokka & Rahzar in Turtles Forever.
In the 2003 animated series, the mutagen was created by the Utroms while they were trying to create a Teleportal machine that will allow them to escape Earth back to their homeland after they crash-landed on Earth. Disguised as the technology company T.C.R.I. they had established in New York in order to allow the funds acquired through their transactions to return home. They tried to create the machine secretly. However, during its creation, the waste turned into a mutagen, with their attempts to dispose of it resulting in a canister falling off the back of a truck and mutating the turtles and Splinter when it fell into a sewer and splashed all over them. Much like in the Mirage Comics, the mutagen causes explicit growth, both physically and intellectually. Among the victims were the turtles, Leatherhead and Splinter.
In the 25th Anniversary Special of the franchise, Turtles Forever. There were small capsules of what appeared to be purple mutagen which 88 Donatello took possession of from the 1988 Shredder from his dimension into to the 2003's Turtles. The mutagen was native to their world and still acted in the manner it did there as opposed to the Utrom variety. 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 after coming into contact with a fighting dog in the Purple Dragons' custody. Hun was mutated into a half-turtle which was vaguely similar to Slash. A great deal of mutagen was mass-produced by Ch'rell and Karai. They used it to mutate Foot Ninjas 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.