To investigate the possible enhancing effect of the H-2z haplotype of the New Zealand White (NZW) strain on the production of autoantibodies and renal disease observed in B/W F1 mice, we developed the ZWD/8 strain, a NZW congenic line carrying the H-2d haplotype, produced (NZB X ZWD/8)F1 (B/WD8 F1) mice, and examined the difference in several immunological abnormalities between the B/W F1 (H-2d/H-2z) and the B/WD8 F1 (H-2d/H-2d) mice. In comparison with B/W F1 mice, the B/WD8 F1 mice showed markedly lower serum levels of the anti-DNA antibodies and the gp70 ICs, and a later onset and a lower incidence of proteinuria with a lower mortality. In contrast, there was no significant difference in the incidences and the amounts of both natural thymocytotoxic autoantibody and anti-erythrocyte autoantibody between these two hybrid strains. Further, the serum levels of IgG and IgM in B/WD8 F1 mice were as high as those in B/W F1 mice. These findings indicate that the gene(s) that is within or closely linked to the H-2 complex of NZW strain specifically acts to intensify the levels of anti-DNA antibodies and gp70 ICs, and to promote the severity of renal disease in B/W F1 mice. This gene may play a role in the class conversion of anti-dsDNA antibodies from IgM to IgG.