The antitoxic potency of crude Crotalus durissus terrificus serum against crotalic venom is similar to that of a standard horse anticrotalic serum in protecting mice against 4 LD50, while the potency of Bothrops neuwiedii serum is 20% of the latter. Failure to form precipitin lines in immunodiffusion tests suggests that the antitoxic factors present in the sera from both species are not immunoglobulins. It is, therefore, probable that crotoxin is not neutralized by an antigen-antibody reaction, but rather by formation of inactive complexes with specific serum components. Resistance to the venom is not reciprocal, since specimens of C. d. terrificus die after the injection of similar amounts of B. neuwiedii venom, which are tolerated by the homologous species.