The glycoprotein localization of the platelet binding site for the Fc IgG has been the subject of debate. We attempted to resolve this issue by relating the binding of radiolabeled IgG immune complexes composed of heat-aggregated IgG to platelets from healthy individuals; an individual with Bernard-Soulier syndrome lacking glycoproteins IIb and IX; and a patient with Glanzmann's thrombasthenia lacking glycoproteins IIb and IIIa. The binding of IgG complexes to platelets was determined by measuring the specific binding of radiolabeled heat-aggregated IgG to washed platelets in a plasma-free mileu. 125I aggregated IgG bound to normal platelets in a saturable and concentration-dependent fashion. Specific binding could be inhibited by a 50-fold excess of purified Fc, but not by F(ab')2. Identical binding curves were obtained by using platelets from a patient with Glanzmann's thrombasthenia and a patient with Bernard-Soulier syndrome, indicating that the platelet Fc receptor is not carried on glycoproteins Ib, IIb, IIIa, or IX. We then measured the binding of radiolabeled detergent-solubilized platelets to IgG fixed to a solid matrix. A 40-kD platelet fragment bound to the immobilized IgG following passage across a density gradient. Confirmation of the Fc specificity of the interaction was shown by inhibition of platelet glycoprotein binding by excess IgG or purified Fc but not F(ab')2. The electrophoretic mobility decreased slightly after reduction, which indicated the existence of at least one intrachain disulfide bond. Treatment with high salt solutions or urea did not solubilize the receptor, which indicated that it was an integral protein. Enzyme studies showed that the platelet Fc receptor was not digested by neuraminidase, but neuraminidase treatment altered mobility by about 3%. In addition, treatment of platelets with trypsin or pronase did not affect its function as measured by the binding of 125I-IgG aggregates to enzyme-treated platelets, but did prevent its detection when using radioimmunoprecipitation studies. The platelet Fc receptor is a 40-kD, integral protein without interchain disulfide bonds.