Fc gamma R expressed by mouse mast cells were characterized as functional binding sites, as membrane proteins, and as products of the two genes known to encode murine Fc gamma RII. Peritoneal mast cells, bone marrow-derived mast cells (BMMC), and the mastocytoma cells P815 were found to bear trypsin-resistant, 2.4G2+, low-affinity receptors binding mouse monoclonal IgG1, IgG2a, and IgG2b, i.e., Fc gamma RII. BMMC and P815 Fc gamma RII appeared as heterogeneous membrane proteins that, when deglycosylated, had m.w. corresponding to those of the three known products of the alpha and beta Fc gamma R genes, and differed by their respective contents in BMMC and P815 cells. Heterogeneous Fc gamma R transcripts were also found in BMMC and in P815 RNA. P815 cells contained alpha, beta 1, and beta 2 Fc gamma R transcripts, whereas BMMC contained alpha and beta 1 Fc gamma R transcripts. These data disclose an unexpected molecular heterogeneity of murine mast cell Fc gamma R. Although they appear as a single population of receptors when viewed by external ligands, mast cell Fc gamma R comprise three Fc gamma RII subtypes, encoded by the three known transcripts of the alpha and beta Fc gamma R genes, and differing by their intracytoplasmic portion. The different distributions of Fc gamma RII transcripts and corresponding Fc gamma RII subtypes in different types of mast cells may be determinant for triggering the various biologic activities of these cells.