The binding properties, solubilization, and sensitivity to enzymatic treatment of the Fc receptors of guinea pig peritoneal macrophages were studied. The receptors on the cell surface were saturable with soluble immune complexes, and the binding showed linearity in the Scatchard plot. The observed linearity probably reflected preferential binding of the immune complexes of relatively large sizes. When receptors on the cells were operationally saturated, the number of antibody molecules in the cell-bound complexes was calculated to be 6.5 × 105/cell. By inhibition test on the binding of 125I-labeled soluble immune complexes to macrophages, receptor activity was found in the 20,000 × G supernatant fraction of sonically disrupted macrophages, which was precipitable at 100,000 × G. Treatment of the 20,000 × G supernatant or intact cells with Nonidet P-40 solubilized Fc receptors. In the presence of detergent, these receptors existed as molecules with a Stokes radius somewhat larger than that of IgG but they reaggregated when the detergent was removed. Treatment of the sonic lysate of macrophages with phospholipase C revealed that Fc receptor activity on 100,000 × G-percipitable cell fragments was lost as indicated by the binding inhibition test. The loss of activity was shown to be not due to release of the receptors from the cell fragment. By using surface-labeled macrophages it was demonstrated that the activity of the receptors was restored by treating the phospholipase C-inactivated cell fragments with Nonidet P-40 as indicated by direct binding of solubilized receptors to insoluble immune complexes. This restoration of activity was probably brought about by binding of detergent to the receptors. These results suggest the possibility that Fc receptors on cell membranes require the structural support of phospholipids for their activity.