Fragments of the alpha-toxin of Clostridium perfringens have been produced using genetic manipulation techniques. Antibody which cross-reacted with the alpha-toxin was induced after immunization with fragments representing the N- (Cpa 1–249) and C-terminal (Cpa 247–370) domains of the toxin. Smaller fragments of the alpha-toxin did not induce cross-reacting antibody. Anti-Cpa 1–249 serum neutralized phospholipase C activity but not haemolytic activity of the toxin. Anti-Cpa 247–370 serum neutralized both the phospholipase C and haemolytic activities. Only immunization with Cpa 247–370 induced protection against the lethal effects of the toxin. Immunization with Cpa 247–370 also provided protection in a mouse model against at least 10 LD 100 doses of C. perfringens type A. This result confirms the essential role of this toxin in the pathogenesis of gas gangrene.