The sera from two patients with unexplained anemia gave a pattern of agglutinationin vitrowhich was the converse of that described for Rh‐like antibodies associated with autoimmune hemolytic anemia. These sera reacted with not only Rhnullred blood cells but also U negative, Bombay and other cells lacking in usual agglutinogens. They appeared to contain antibodies directed against antigenic structures which are either absent or covered on the surface of red blood cells of the more usual type