Elevated levels of platelet-bound IgG (PA-IgG) are a feature of autoimmune thrombocytopenia (ATP), but it is well documented that this does not occur in all cases. This has led us to investigate the role of platelet-bound IgM (PA-IgM) in these patients using a quantitative enzyme-linked immunosorbent assay (ELISA). Forty-five determinations of PA-IgM and PA-IgG were done on 24 patients with ATP. Elevated levels of PA-IgM were found in 93.3% of the determinations, while PA-IgG was elevated in only 71.7%. In 64.4% of determinations, both were elevated. Elevated PA-IgM or PA-IgG alone occurred in 28.9% and 6.7% of determinations, respectively. These results show the hitherto unrecognized frequent involvement of PA-IgM in ATP and suggests that a complex interrelationship exists between the two immunoglobulin classes in ATP. Some of the possibilities that might explain this interrelationship are discussed.