Head back to the Medical Bay and insert the Decontamination Agent into a glass case on the wall. The computer will now tell you that it waits for a subject.
As a subject, you will need a Megaton. In case there isn't any, simply wait till a round when it will spawn. Now lead him in the Medical Bay, under the Wechsler decontamination room, and shoot the creature untill it splits into two. You can also harm the Megaton till it almost splits in two, cease fire, lure him under the Wechsler, and shoot the final shots on it.
When it splits in two, the creatures will be sucked into the decontamination room. You need to make sure to lure both creatures in the machine.