The transmission of murine leukemia virus (MLV) to hybrids between AKR and Fv-1(b) mice was studied in order to evaluate the effect of the Fv-1 gene on endogenous MLV infection and to attempt to determine if the genetic loci contributed by AKR carry viral genetic determinants. Fv-1 was shown to have a marked suppressive effect on time of appearance of detectable infectious virus and on the titers attained in vivo, but did not affect the ability of the cells to produce virus in vitro after induction with 5-iododeoxyuridine. The host range type of the virus detected in the hybrid mice was almost always of the type carried by AKR, although the low-virus Fv-1(b) parents carry the genome of a different host range type. This finding provides strong, but not conclusive, evidence that the virus-inducing loci of AKR contain MLV genetic determinants.