A simplified and sensitive enzyme immuno-assay employing microtiter plates as the solid phase carrier for detection of circulating platelet antibodies and bound antiplatelet IgG has been described. The assay was done using fresh as well as frozen platelets and commercially available peroxidase labeled anti-human IgG. The sensitivity of the enzyme immuno-assay was found similar or superior to that of the platelet suspension immunofluorescence test and superior to the lymphocyte cytotoxicity test and the platelet complement fixation test. The use of platelets frozen in the wells of the microtiter plates (stored for up to 17 months at -20 degrees C without loss of antigenicity) facilitates the performance of the test. In addition the small volumes of sera and platelets needed in using microtiter plates makes the assay particularly suitable in testing platelets from thrombocytopenic patients. In one of 31 sera from normal Zwa-negative donors a weak anti-Zwa (P1A1) was found only detectable by enzyme immuno-assay. Alloantibodies were found in 14 of 23 patients suffering from non-haemolytic transfusion reactions. All of three patients with post-transfusion purpura had anti-Zwa antibodies in serum. Anti-Zwa antibodies were demonstrated in the sera from all of 9 Zwa-negative mothers, who had given birth to children with alloimmune neonatal thrombocytopenia. In 8 of 12 Zwa-positive mothers alloantibodies of other specificities were found in the serum. In one case the antibody was a known anti-Baka. Six of 7 patients with autoimmune thrombocytopenic purpura had values of platelet-bound IgG exceeding the normal range and circulating platelet antibodies were seen in 4 patients.