The first two cases of post transfusion purpura (PTP) due to recognized antibodies against the platelet specific antigen Zwb (P1A1) are described. In both cases, the antibodies were detectable in the platelet enzyme immuno-assay (ELISA), while the platelet specific alloantibodies could be ascertained neither in the platelet suspension immunofluorescence test (PSIFT) nor in the platelet fixation test (CFT) due to coexisting potent multispecific HLA-antibodies. The two sera reacted in parallel (coefficient of correlation, r = 0.87; P less than 10(-5) and the reactions of the two sera were negatively associated with the Zwa-antigen (P = 0.0002 and 0.003, respectively). Thus, PTP may be caused not only by anti-Zwa (-P1A1) but also by anti-Zwb.