We did crossmatching in 220 transfusions of single donor platelet concentrates using the purchaseable solid phase immunoassay Capture-P (Immucor, Rödermark, FRG), checking posttransfusion response by calculating the corrected count increment (CCI). Our results show that posttransfusion response in transfusions of ABO major incompatible platelet concentrates is virtually reduced only in those transfusions accompanied by a positive crossmatch in Capture-P. As Capture-P can detect IgG antibodies only, positive test results could well be due to a reaction of recipient immune isoagglutinins with ABH antigens expressed on donor platelets, therefore.