To determine the biochemical basis for the two distinct IgG subclass binding specificities on mouse macrophage-like cell lines, Fc-binding proteins were isolated from P388D1 and J774 cells by affinity chromatography using immobilized IgG. Similar IgG subclass-specific Fc gamma-binding proteins were identified on both cell lines. The Fc-binding proteins were characterized by their IgG subclass binding specificity, size, charge, and trypsin sensitivity. Using immobilized IgG1, a protein was isolated with an average m.w. of 65,000 and a pI range of pH 4.7 to 5.8. This protein could also be isolated, although in reduced amounts, with immobilized IgG2b. Two proteins with average m.w. of 70,000 and 60,000 and a pI range of 3.8 to 4.7 were isolated with immobilized IgG2a. The 60-kd protein appeared to be derived from the 70-kd protein. The IgG1/IgG2b Fc-binding proteins were resistant to trypsin, whereas the IgG2a Fc-binding proteins were sensitive to trypsin. In IgG-binding studies with the isolated proteins, the IgG2a Fc-binding proteins continued to exhibit restricted binding specificity for the IgG2a subclass; however, the IgG1/IgG2b Fc-binding proteins showed a broader specificity and would rebind to IgG1, IgG2a, and IgG2b. The IgG-binding properties and trypsin sensitivity of these Fc-binding proteins were similar to the properties of the IgG binding sites on the intact cells. These data indicate that the IgG subclass-specific binding sites on macrophage-like cell lines are due to the presence of distinct protein molecules on the cell surface.