Mixed passive hemagglutination (MPHA) test was established using platelet-supernatant and indicator cells coated with purified antihuman IgG antibodies. The supernatant was shown to contain HLA A and B antigens, platelet alloantigens and blood group A, B and H antigens released spontaneously from the platelets. Unlike the MPHA with platelet monolayer, the in vitro-formed immune complex (IC) or IC containing pathologic sera did not give positive reactions in the MPHA with platelet supernatants presumably due to relatively low Fc receptor activities of the supernatants. By means of this test, HLA A or B antibodies were demonstrated in sera of 32 (44%) of 72 patients with multiple platelet transfusions including ‘cytotoxicity negative absorption positive’ sera. This test would provide a sensitive procedure for cross-match of sera of recipients of platelet transfusion and screening of HLA A and B typing s
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