The importance of various components of the plasma membrane in the generation of the immune response has long been recognized. The identification and characterization of functional membrane receptors are essential for an understanding of the molecular basis of the immune response. The Fc receptor can be operationally defined as a site on the plasma membrane which is capable of binding the Fc portion of IgG. The Fc receptor has been reported on both lymphoid and nonlymphoid cell types. The functional significance of the receptor on these cell populations, as well as the biochemistry of the receptor, is not well established and is currently an important area of investigation. In this review, the Fc receptor will be discussed, primarily in human and murine lymphoid and macrophage cell populations, as a model of membrane immunoreceptors. The review will be divided into four sections. In the first section, consideration will be given to the binding assays used to study Fc receptor-ligand interactions and the class and subclass specificity of the receptor. The possibility of receptor-receptor interaction and quantitation of receptor sites will be discussed. The second phase of the review will be concerned with the fractionation and biochemistry of this receptor system, emphasizing the differences reported in receptor composition from different cell sources. In the third section of the review, the biologic significance of the receptor will be examined. The importance of the Fc receptor in opsonization and phagocytosis by mononuclear phagogytes, B-cell activation, antibody-dependent cell cytotoxicity, and the possible relationship of Fc receptors to the histocompatibility-linked immune response genes will be discussed. Finally, in the fourth section, the possible mechanisms of receptor regulation will be considered. Cell fusion experiments which have been performed and the expression of receptor activity on the hybrid cell clones will be discussed. Receptor regulation at the physiologic level will include the effects of cyclic nucleotide levels and cell cycle on receptor expression. It is the intent of this review to provide a detailed analysis of the Fc receptor and to emphasize its importance in microbiology, cell biology, and immunology.