Forssman hapten isolated and purified from sheep erythrocytes showed a single spot on the thin layer chromatography. The hapten preparation when immunologically tested by hemolysis inhibition tests showed constant inhibitory activity. The antibodies prepared also showed similar hemolytic activity as those produced with sheep erythrocytes. Acid hydrolysis of the hapten of the hapten indicated the presence of galactosamine, galactose, glucose and ceramide. The terminal group of the hapten was identified to be α N acetyl D galactosamine both by the inhibition of agglutination of human A cells with anti A serum and by the hydrolysis of the end group with a specific enzyme from Achatina fulica. The next sugar molecule was identified as β D galactose both by inhibition of agglutination of human B cells with anti B serum and by the estimation of D galactose liberated by the enzyme, β D galactosidase of rat intestine. The remaining portion of the hapten was identified to be β D galactose O β D glucose ceramide (cytolipin H). The ceramide was identified to be lignocerylsphingosine. Hence, Forssman hapten of sheep erythrocytes is assigned the structure: α N acetyl D galactosamine (1→3)OβD galactose (1→4)OβD galactose (1→4)OβD glucose(1→1)ceramide.