Many methods have been described to identify platelet antibody, but they are either not very sensitive or too complex for general use. Therefore, we have developed an enzyme immunoassay for the detection of platelet antibodies in serum. The method involves incubating platelets with serum antibody; any attached antibody is shown by the addition of an enzyme (alkaline phosphatase) labeled anti-human IgG, followed by assay of the enzyme reaction with its substrate. The reaction product is indicated by a color change, which is proportional to the antibody concentration. Assay conditions such as the use of paraformaldehyde fixed versus unfixed platelets, conjugate dilutions, and substrate concentration and incubation time were investigated. Positive results were obtained in 16 of 19 sera of patients with various diseases including 2 of 4 patients with idiopathic thrombocytopenic purpura, 2 of 2 with post-transfusion purpura, 2 of 3 with neonatal purpura, and all 9 polytransfused patients. Sensitivity and specificity were 84% and 98%, respectively. Also, enzyme linked immunospecific assay (ELISA) was found to be superior to the lymphocytotoxicity (LCT) and platelet immunofluorescence test (PIIFT) for platelet antibody identification.