The occurrence of lupus nephritis in (NZB x NZW)F1 mice appears to depend on the action of at least two dominant or co-dominant genes (at least one gene from each parent) as neither of the inbred parental strains shows the disorder. Identifying affected animals by antemortem determinations of renal function, using improved methods of measuring proteinuria and renal clearance, we have studied the incidence of the renal disease in 230 (NZB x NZW)F1 x NZW backcross mice. The incidence was 49-6% which indicates that NZB strain contributes only one gene, or cluster of closely linked genes, to the renal disorder of the F1 hybrid. The gene(s) must be dominant or co-dominant, as it expresses its effect in the heterozygous state. Study of the H-2 status of the backcross mice showed a loose linkage of the NZB renal disease gene(s) to the D end of the H-2 complex, the crossover frequency being 32-6+/-3-1%.