Abstract Forssman hapten capable of precipitating specific antibody was isolated from stromata of erythrocytes of sheep. The product was found to be relatively pure. It melted with decomposition at 203–208 °, contained 2.92% nitrogen and less than 0.1% phosphorus, and possessed a molecular weight in the neighborhood of 1000. Galactose, chondrosamine, and lignoceric acid were tentatively identified in the acid hydrolyzate of the hapten. A basic component was not characterized.