A simplified, sensitive, solid-phase radioimmunoassay employing 125I- staphylococcal protein A has been developed that is capable of detecting bound antiplatelet IgG as well as serum auto-, allo-, and drug-dependent antiplatelet antibodies. The simplified assay employs a ratio of test over control platelet counts per minute (cpm) for detection of positive results. All reagents are commercially available. The assay can be performed with as little as 10(6) washed platelets (10 microliters of whole blood) that have been stored for as long as 8 wk at 4 degrees C in microtiter plates. The assay time, employing stored platelets, is 4 hr. Bound platelet IgG is positive in 93% of 46 thrombocytopenic patients with autoimmune disease and correlates inversely with their platelet count, r = -0.65, p less than 0.001. The ability of this assay to detect serum antibody was studied with a rabbit anti-human platelet antibody capable of giving optimal immunoprecipitation with solubilized platelet membranes at a tier of 1:10. The present assay increases the sensitivity of antibody detection 256-fold to a titer of 1:2560. Human serum antiplatelet membrane antibody was positive in 2 of 2 patients with anti-PLA-1 antibody (titers of 1:256 and greater than 1:64); 7 of 12 multiply transfused patients who were refractory to platelet transfusion (2 had titers of greater than 1:256 and greater than 1:32); 5 of 19 patients with autoimmune thrombocytopenic purpura (2 had titers of 1:64 and 1:32); and 10 of 14 patients with clinical histories of drug-dependent antiplatelet antibody (2 had titers of 1:1280 for quinidine and 1:384 for phenazopyridine).