Immunological relationships among antihemorrhagic factors (AHF) present in the plasma of different snakes and a mammal (opossum) were studied. Antibodies were prepared against purified Bothrops jararaca and Didelphis marsupialis aurita (opossum) AHF. The antigen-antibody reaction was determined by direct ELISA and by the competition of homologous antigen-antibody reaction with a heterologous competitor antigen. Plasmas from several snakes and from opossum were used as antigens. Anti-AHF (B. jararaca) reacted only with snake plasmas. Antibody against opossum AHF was detectable only when the homologous antigen was used. Differences in the reactivity of snake plasmas with antibodies against B. jararaca were demonstrable only when a competition assay was used: B. alternatus showed a reaction pattern similar to that of B. jararaca while Crotalus durissus terrificus and the nonvenomous snake, Wanglerophis merremii, presented reduced reactivity.