Although the presence of antiplatelet antibody in idiopathic thrombocytopenic purpura is generally accepted, information on the quantity synthesized, the production site (or sites), and the degree of binding to platelets is sparse. Methods described here allow an evaluation of these characteristics. Mean in vitro IgG production by spleens of patients was sevenfold greater than control tissue; a portion of the synthesized IgG from 19 of the 20 spleens of patients tested showed significant binding to target platelets. The maximum amount of IgG binding to target platelets and the percentage of platelet-specific IgG were determined in seven cases. Values for maximum binding averaged 1331 ng of IgG per 109 platelets, or 5008 molecules per platelet. Mean percentage plateletspecific IgG was 2.2 per cent, corresponding to a daily splenic production of 627 μg—a quantity that would "maximally sensitize" 2.9 times the average daily platelet production. We conclude that the spleen in idiopathic thrombocytopenic purpura is one important site of antiplatelet antibody production. (N Engl J Med 291:812–817, 1974)