An enzyme linked immunosorbent assay (ELISA) method for the detection of antibodies to platelets and leucocytes is presented. The method can be used for large numbers of samples. The method is objective when photometers are used. Approximately 50% of all cases of possible autoimmune thrombocytopenic purpura (A.T.P.) and unexplained neutropenia showed positive results. The results obtained using ELISA and standard tests for leucocyte and platelet antibodies are compared. The ELISA tests may also detect immune complexes.