The in vivo distribution of labeled Clostridium perfringens epsilon toxin after i.v. administration to mice was investigated. High amounts of radioactivity were found in the kidneys and the brain, and small amounts were in the heart, lungs, liver and stomach. On the other hand, the prior administration of epsilon prototoxin resulted in significant inhibition of the uptake of the radioactivity in the brain, but no effect in the other organs tested. The labeled prototoxin or toxin was dose-dependently accumulated in the brain after i.v. injection. However, the prototoxin inhibited the uptake of the toxin injected within 10 min after the administration of the prototoxin. The prototoxin also inhibited the uptake of labeled bovine serum albumin caused by the toxin in the brain. In the present paper, these data show that the toxin is specifically bound to the brain of mice.