With data on 91 alloimmunized thrombocytopenic patients and 389 donor-recipient pairs matched or selectively mismatched for HLA antigens, it was observed that ABO incompatibility significantly reduced the effectiveness of platelet transfusions. The mean 24-hr recovery of platelets from histocompatible donors and from donors selectively mismatched for cross-reactive HLA antigens was decreased by approximately 23% if the donor typed for blood group A and/or B not found in the recipient. Thus, the reduction in platelet recovery associated with ABO incompatibility is not of a magnitude that would contraindicate transfusion of ABO-mismatched platelets.