Three common proviral integration sites, Fim-1, Fim-2/c-fms, and Fim-3, have been described in mouse myeloid leukemias induced by the Friend murine leukemia virus. The nature and function of Fim-1 and Fim-3 are still unknown since no transcript from these loci has been detected so far. To identify these two loci, we undertook their chromosomal localization using restriction fragment length polymorphism detected between C57BL/6 mice and the wild-derived inbred strain of Mus spretus. Using interspecific backcross analysis, we mapped Fim-1 to mouse chromosome 13 and Fim-3 to mouse chromosome 3. Interestingly, Fim-3 is tightly linked to Evi-1, another common integration site of ecotropic virus involved in another model of mouse myeloid leukemogenesis. Fim-2 spans the 5' end of the c-fms gene, which encodes for the macrophage-colony-stimulating factor receptor. We located the c-fms gene on the D band of chromosome 18 by in situ hybridization.