Mutagen is one of the meters in Area 51 which is acquired in the level One of Them. It keeps Ethan turned as a mutant and to increase the dwindling meter, you have to engage in mutant claw melee combat. Some players would of thought that the way to fulfill the objective in One of Them would to not engage in combat and make the meter go dry. However you must kill a group of leapers and go up a ladder and find a tube that gives you a choice between mutant or human. Mutagen also comes in syringes like health.