Soluble blood group substances, isolated from the red blood cells of humans, baboons, and vervet monkeys by ethanol extraction, possessed serologically active specificities for the following antigens: A, B, H, Lea, LebL, P, P1, Pk, and I. Human red blood cells lacking any of these specificities by the direct hemagglutination test also lacked the related antigens in their soluble extract. The only exception was in ‘Bombay’ Oh cells, from which soluble H substance could be readily isolated. Soluble substances obtained from baboon and vervet monkey red blood cells, which lack the human variety of A, B, and H antigens on their red blood cells, inhibited both human and lectin anti‐H reagents. The detection of ‘hidden’ H activity in Oh cells will pose some important questions regarding membrane characteristics and the role of immune sur