Estimates of the number of IgG molecules bound to normal platelets have ranged from several hundred to several tens of thousands. The lower estimates were generated from direct binding assays and stoichiometric assumptions. The higher values derive from competitive binding assays, in which platelet-associated IgG (PAIgG) is calculated from a standard curve using soluble IgG standards. Using a kinetic-ELISA (enzyme-linked immunosorbent assay) antiglobulin assay, we measured normal platelet IgG to be 21,200 +/- 9,400 molecules per platelet when a competitive assay and soluble IgG standards were used. Direct measurement of bound antiglobulin by kinetic-ELISA and stoichiometric assumptions yielded a measurement of 259 +/- 117 IgG molecules per platelet. Soluble IgG and PAIgG are not comparable in their ability to bind anti-IgG. Disparities in estimates of normal PAIgG are probably due to methodological differences. The estimate most likely to be correct is several hundred IgG or less per normal platelet.