The amount of IgG present on the surface of platelets (platelet-bound IgG, or PBIgG) was measured using a quantitative antiglobulin consumption assay developed by Dixon et al. (1975). Normal platelets or platelets from patients with thrombocytopenia due to a decreased production had less than 10 X 10(-15) g of IgG/platelet. Studied were 364 blood samples drawn from 196 patients; 173 patients had idiopathic thrombocytopenic purpura; 65 (93 %) of 70 in the acute phase of their disease had elevated PBIgG (values up to 100 times greater than the normal); 98 (76.5 %) of 128 blood samples from patients during a chronic phase had abnormally high PBIgG but the levels of IgG were generally lower than those observed in the acute phase; 75 (75 %) of 100 samples from patients in remission did not have pathologically increased PBIgG. Usually when the platelet count improved (after prednisone therapy or after splenectomy), PBIgG decreased. The PBIgG determination was not useful for predicting the response to prednisone therapy or the further evolution of the disease, except when PGIgG remained elevated after high-dose prednisone therapy; this generally implied a poor prognosis. PBIgG have also been found elevated in other diseases such as lupus erythematosus and in one case of chronic lymphocytic leukemia. This assay may be of a great clinical interest, especially for the diagnosis of ITP.