We investigated whether the platelet-membrane surface carries IgG allotypic antigens and whether these determinants may be important in platelet transfusion therapy. Using a hemagglutination inhibition assay, we showed that the G1m IgG allotypes (a, x, f) and K1m and K3m light-chain allotypes are expressed on the surface of platelets, whereas G3m allotype antigenic determinants were not detectable. In 146 multitransfused thrombocytopenic patients, 35 (24%) patients were found to have antiallotypic antibodies. To study the effect of antiallotypic antibodies on platelet transfusion outcome, patients received platelet transfusions from donors, either positive or negative for the IgG allotype to which patients were immunized. Of the 19 antigen-positive and 19 antigen-negative platelet transfusions given, respectively, the mean platelet count increments at 1 hour were 8,402 +/- 6,402 +/- 6,721 (1 SD) and 9,799 +/- 5,559 (1 SD) P less than .2. Transfusion reactions were not more common when antigen-positive platelet transfusions were given. Despite the presence of IgG allotypic determinants on platelets, allotypic antibodies do not decrease platelet transfusion recovery. Furthermore, passive administration of plasma containing IgG allotypes to patients with antiallotypic antibodies does not lead to innocent bystander-mediated platelet destruction.