Simian virus 40 (SV40)-transformed human cells (LN-SV) were fused with BALB/c peritoneal macrophages (BALB/c X LN-SV) and with C57BL peritoneal macrophages (C57BL X LN-SV) and hybrid clones, all of which had segregated human chromosomes and contained the entire complement of mouse chromosomes, were isolated. All 15 BALB/c X LN-SV hybrid clones were producing varying titers (10 to 10(6) plaque-forming units/ml) of B-tropic murine leukemia virus, whereas none of the nine C57BL X LN-SV hybrid clones was producing detectable ecotropic murine leukemia virus.