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 Hero Turtles. It is responsible for mutating, among others, Splinter and the four Hero Turtles, and villain pairings such as Bebop and Rocksteady as well as the Punk Frogs.
During the Teenage Mutant Hero Turtles Introduction set of comic strips, April O'Neil tells the story of how Hamato Yoshi came to live in the Sewers of New York City. This includes the tale of the young boy who loses his pet turtles down a sewer grate and how they, along with Yoshi, were exposed to a glowing liquid mutagen that changed them all.
In Holiday in Florida!, Krang tricks the Turtles into thinking they've won a free vacation in Florida. Once they are there, Bebop and Rocksteady lure them into the swamps and use the mutagen given to them by Krang to mutate four frogs into giant frogs. At first, the Turtles think they are seeing their buddies, the Punk Frogs, but quickly realize these frogs are much larger. Fortunately, the real Punk Frogs arrive and help to fight off the giants as well as Bebop and Rocksteady. Making another attempt to defeat the Turtles, Krang sends two Stone Warriors from Dimension X to the Florida. They are making short work of the Turtles when April and Casey Jones arrive to assist. Casey knocks out the Stone Warriors and then April finds the canister of mutagen the two baddies brought along. Donatello sprinkles it on the two Stone Warriors to see what will happen. They grow larger, but also mutate into the last creatures they touched: Bebop and Rocksteady. Fearing that two sets of Bebop and Rocksteady is too much for any universe, Krang sends the Stone Warrior pair back to Dimension X.