The relationship of B cell hyperactivity and retrovirus expression to other autoimmune traits were examined in recombinant inbred (N X 8 RI) lines derived from NZB and C58 progenitor strains. Although both NZB and C58 mice expressed high levels of xenotropic virus, the RI lines segregated in virologic phenotype, as high or low expressors of the endogenous virus. The expression of the C58-derived ecotropic virus occurred in only one-half of the RI lines, and its expression in the remaining lines of mice appeared to be suppressed by the NZB-derived allele at the Fv-1 locus. The inheritance of B lymphocyte abnormalities of the NZB progenitor strain was investigated by studying spontaneous and SRBC-induced production of IgM by the spleen cells of the RI lines. These two phenotypes of B cell hyperactivity were found to be determined by independently segregating genes and they were not linked to immunoglobulin structural gene loci. The strain distribution patterns of virus expression and B cell hyperactivity in the RI lines did not match with each other or with the inheritance patterns of other immunologic abnormalities, such as defective AMLR and production of autoantibodies.