FcγRIIB are single-chain low-affinity receptors for IgG that bear an immunoreceptor tyrosine-based inhibition motif in their intracytoplasmic domain and that negatively regulate immunoreceptor tyrosine-based activation motif-dependent cell activation. They are widely expressed by cells of hematopoietic origin. We investigated here whether FcγRIIB could also negatively regulate protein tyrosine kinase receptor (RTK)-dependent cell proliferation. As an experimental model, we used growth factor-dependent mast cells that constitutively express FcγRIIB and c-kit, an RTK prototype. We found that anti-c-kit Abs mimicked the effect of stem cell factor and induced thymidine incorporation in FcγRIIB−/−, but not in wild-type (wt) mast cells unless FcγRIIB were blocked or anti-c-kit F (ab′) 2 were used. When coaggregated with c-kit by intact Abs in wt mast cells, FcγRIIB inhibited thymidine incorporation, as well as cell proliferation, and inhibition was correlated with an arrest of cells in G1 during the cell cycle. The coaggregation of c-kit with FcγRIIB did not affect ligand-induced c-kit phosphorylation and induced the tyrosyl-phosphorylation of FcγRIIB, which selectively recruited the Src homology 2 domain-bearing inositol 5-phosphatase SHIP. Our results indicate that IgG Abs to growth factors or growth factor receptors may control RTK-dependent proliferation of a variety of cells that express FcγRIIB.