AbstractPlatelet antibodies were looked for in 47 patients with autoimmune thrombocytopenia using a modification of the enzyme‐linked assay previously described. Surface‐bound antibodies measured as increased platelet‐associated IgG were found in 32 (68%) of the patients. After incubation in test sera, the platelet‐associated IgG of normal donor platelets was significantly increased in 27 of the 47 patients (57%), thus demonstrating the presence of platelet antibodies free in their sera. 6 patients had antibodies only in the serum without any elevation of their platelet‐associated IgG. When both tests are evaluated together no antibody was detected by either the direct or the indirect test in 9 of the 47 patients (19%) studied. The technique used is described and the interpretation of our results