Summary: Antispermatozoal sera react with spermatozoa and not with serum of the same species; antisera against serum react with serum, and not with spermatozoa. Isoimmunization of female rabbits and female white rats was successful. Spermatozoa are dominantly species-specific; antisera against spermatozoa reacted best with homologous sperm cells. Nevertheless distinct cross-reactions occurred with heterologous spermatozoa. Isoantisera reacted like heteroantisera. It is therefore possible to differentiate between spermatozoa of different species by means of antispermatozoal isosera. Cross-reactions are stronger in closely related species, but they also occur between more distantly related species. They are distinctly positive between the spermatozoa of bull and man. Blood-group substances are not causes of these cross-reactions.