This locus controls resistance and susceptibility of cells in tissue culture to infection by mink cell focus-forming murine leukemia viruses. The allele Rmcf^r determines resistance and occurs in strains DBA/1, DBA/2, and CBA/Ca; the allele Rmcf^s determines susceptibility and occurs in strains AKR/J, C57BL/6, BALB/c, CBA/J, NFS, NZB, 129/J, and many others. Heterozygotes are as resistant as the resistant parent or nearly so. Rmcf is different from and independent of Fv1, a locus that controls susceptibility to infection by ecotropic viruses. Rmcf is located on Chr 5 close to Hm near the centromeric end (J:7108). Rmcf^r protects (AKR x CBA/Ca)F1 and (AKR x DBA/2)F1 hybrids from development of spontaneous thymic lymphomas and reduces the incidence of MCF-induced thymic lymphomas (J:7175). It also reduces susceptibility of cells of Sxv^s/Sxv^r mice to exogenous xenotropic viruses (J:7951). In addition, in strains susceptible to Friend virus-induced erythroleukemia, a condition thought to be due to the replication of MCF virus, Rmcf^r increases resistance to the virus-induced erythroleukemia. It may cause resistance by coding for or regulating the production of an MCF-related envelope glycoprotein that blocks the receptor for MCF viruses (J:8074). This conclusion is reinforced by the findings of Buller et al. (J:8497), who showed that the Rmcf^r allele contains an endogenous MCF gp70 env gene and that the Rmcf^s allele, at least in some strains (C57BL/6, CBA/J, and A/WySn), contains a xenotropic gp70 env gene. Presumably the MCF gp70 inhibits exogenous MCF infection in vitro by a mechanism of viral interference.