Formaldehyde-fixed human erythrocytes were extracted with sodium dodecyl sulfate and with three other solvent systems, at least two of which are known to remove glycolipids quantitatively. The extracted cells possessed the ability to absorb the ABO blood group-specific antibody at about one-third the level of unextracted cells. Treatment of fresh cells with pronase also reduced the ability of the cells to absorb the antibody, further supporting the presence of ABO blood group active glycoprotein in the membrane. Trypsinization of red cells, while removing PAS-1 and partly PAS-2, did not lead to any decrease in the activity. Papainization also did not diminish the activity, although PAS-1, PAS-2, and PAS-3 were removed from the cells. Thus, both glycolipid and glycoprotein contribute to ABO antigens of erythrocytes. Also, none of the three major glycoproteins of the membrane bears this activity.