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Antiplatelet antibody in the serum of a fatal case of post-transfusion purpura showed specificity for the PlA1platelet isoantigen in that the antibody was adsorbed by all PlA1-positive platelets tested but not by PlA1-negative platelets. However, some PlA1-positive platelets that fixed complement with the reference anti-PlA1did not fix complement with this patient's antibody. When used with these platelets, the patient's antibody competitively blocked the complement-fixing activity of reference anti-PlA1. There was no evidence for lack of antigenic sites on the noncomplement-fixing platelets or for antibodies with more than one specificity in the patient's serum. These studies, therefore, indicated that the complement-fixing activity of different examples of anti-PlA1is variable, and that this variation is probably due to distribution of PlA1antigenic sites on cell surfaces rather than to differences in antigenic structure that affect antibody affinity.