The seven CXB recombinant inbred strains were tested for susceptibility to Friend helper virus (F-MuLV) hematopoietic neoplasms. BALB/c and CXB-H mice develop erythroblastosis after neonatal inoculation with F-MuLV, while C57BL/6 and the six other RI strains develop lymphoma and myelogenous leukemia. This strain distribution pattern is different from that for H-2, Gpd-1 (linked to Fv-1), Fv-2, Rfv-3, and Cv (linked to Rmcf) but the same as that for Bv, the endogenous ecotropic virus of C57BL/6. However, analysis of crosses segregating Bv show that resistance to F-MuLV erythroblastosis is not linked to Bv. Disease-free survival is shortest for BALB/c mice, intermediate for CXB-H and CXB-J, and longest for C57BL/6 and the other RI strains. We conclude: (a) the major C57BL/6 gene for resistance to F-MuLV erythroblastosis is different from previously identified Friend virus restriction loci; (b) latency for F-MuLV leukemias is controlled by more than one gene; and (c) latency and susceptibility to F-MuLV erythroblastosis are not inherited concordantly in the CXB-RI strains.