In order to measure platelet-associated IgG (PAIgG), we devised a solidphase enzyme immunoassay employing a competitive binding of peroxidase-conjugated anti-IgG antiserum between platelets and polystyrene tubes coated with IgG. The amounts of peroxidase bound to the tubes were measured in a spectrophotometer by an enzymatic reaction. This method is highly sensitive, reproducible and can be carried out more simply. The PAIgG values of normal controls averaged 21.6 ± 6.6 (SD) ng/l0τ platelets. 27 (93%) of 29 patients with idiopathic thrombocytopenic purpura (ITP), who had a platelet count of less than 15×104/μ l, had PAIgG values greater than those of controls by 2 SD and averaged 205.5 ± 323 ng. There was a significant inverse correlation between platelet count and PAIgG value of ITP patients. The PAIgG values of patients with aplastic anemia were within normal range.