The Rmcf gene restricts the replication of recombinant murine mink cell focus-inducing (MCF) viruses in cell cultures derived from mice carrying the resistance allele (Rmcf(r)) and may play a role in resistance to retrovirus-induced leukemias in vivo. We have characterized the endogenous gp70 expressed by Rmcf(r) and Rmcf(s) mice with a panel of type-specific monoclonal antibodies which discriminate xenotropic and MCF gp70. Embryo and tail skin cultures derived from Rmcf(r) mice (DBA/2 and CBA/N) expressed gp70 bearing a determinant unique to MCF viruses, whereas cultures from Rmcf(s) mice expressed either no detectable gp70 (NFS/N and IRW) or a gp70 serologically related to a subgroup of xenotropic viruses (C57BL/6, CBA/J, and A/WySn). Studies of progeny embryos derived from a (C57BL/6 x DBA/2) x C57BL/6 backcross established that the Rmcf resistance allele was linked to the expression of the MCF gp70 and that the gene encoding the xenotropic gp70 expressed by C57BL/6 Rmcf(s) mice was allelic with the MCF gp70 from Rmcf(r) mice. These data indicate that the Rmcf locus contains an endogenous gp70 gene having two allelic forms, one of which inhibits exogenous MCF infection in vitro by a mechanism of viral interference.