Summary: Anti-brain sera react with the alcoholic extracts of testicle and anti-testicle sera react with alcoholic extracts of brain. The interaction of brain and testicle is independent of species as anti-brain and anti-testicle sera will react with alcoholic extracts of brain and testicle of different species. Anti-testicle sera, like anti-brain sera, do not react with the alcoholic extract of liver, lung, heart, spleen, kidney, and ovary of either the homologous species or the heterologous species tested. The alcoholic extracts of brain and testicle cannot be distinguished serologically. The alcoholic extract of testicle is iso-antigenic when activated with a foreign protein.