About 12 antigens were demonstrated in saline extracts of homogenized rat brain by immunodiffusion and immunoelectrophoresis. A brain organ antigen was discovered which appears to be species restricted. One antigen was found to be shared by similar extracts of bovine, guinea pig, human or rabbit brains. Immunoelectrophoretic analyses of the brain extracts showed this organ antigen had an α2 or β1 mobility, depending on the species from which it was derived. About nine antigens were species-specific globulins and could be partially separated from one another by diethylaminoethyl cellulose chromatography. All of these antigens were shared by the brain and different rat organs. However, one major antigen was shared by all the rat organs tested.