The mouse Ly-17.2 alloantigen has recently been defined with both conventional and monoclonal antibodies; it identifies a locus, sited on chromosome 1, the products of which were considered to be specific for B cells. Using another Ly-17.2-specific monoclonal antibody (described herein), the tissue distribution of the Ly-17.2 antigen was shown to extend to a subpopulation of T lymphocytes and to neutrophils. This distribution is remarkably similar to that of the Fc receptor for immunoglobulin. Indeed, we now demonstrate that the Ly-17 locus codes for a polymorphism of the Fc receptor, a conclusion based upon (a) an identical tissue distribution of Ly-17.2 and FcR on both normal and tumor tissue; (b) specific inhibition of EA rosette formation by F(ab′)2 fragments of anti-Ly-17.2; (c) inhibition of the binding of the 2AG2 monoclonal rat antimouse Fc receptor antibody by Ly-17.2 antibody; (d) precipitation of an identical series of molecules by our Ly-17.2-specific antibody and by the recognized Fc receptor-specific antibody (2.4G2); and (e) the demonstration by coprecipitation that the Ly-17.2 specificity is present on Fc receptor molecules. The studies suggest that the xenogeneic monoclonal antibody (2.4G2) which recognizes an invariant site on the FcR molecule and the polymorphic site are closely associated. In addition, the studies firmly map a gene coding for or regulating the expression of the FcR to chromosome 1.