Since platelets express both platelet-specific and class I HLA antigens, serum antiplatelet reactivity assessed by most platelet antibody techniques could be due to antibodies with either or both specificities. Flow cytometric analysis of sera for detection of antiplatelet antibody commonly employs a purified platelet preparation as target cells. A method is described for investigating sera for platelet antibodies by flow cytometry using a mixture of platelets and lymphocytes. The mixture of lymphocytes and platelets as target cells has the advantage of confirming the presence of the HLA antibodies in reactive sera. The concomitant use of platelets and lymphocytes treated with citric acid, pH3, or with chloroquine (to remove or alter surface HLA antigens without affecting platelet specific antigens) may further assist in identifying antiplatelet antibodies in alloimmunized patients. These techniques may also be useful in platelet crossmatching procedures.