The present genetic studies were designed to investigate a possible correlation of spontaneous polyclonal activation of B cells with the spontaneous production of natural thymocytotoxic autoantibody (NTA) in female (NZB X NZW)F1 X NZW backcross mice. We analyzed the spontaneous polyclonal activation of B cells by determining the serum level of naturally occurring anti-hapten (dinitrophenyl, DNP) IgM antibodies. Based on comparisons of the magnitude of the serum DNP binding activities among NZB, NZW, the F1, and the backcrosses to NZW mice, we found that the abnormal polyclonal activation of B cells depends largely on the contribution of NZB genomes. Similar observations were made on serum IgM levels in these mice, and there were made on serum IgM levels in these mice, and there was a highly significant correlation between these two measures in the backcross mice. Despite striking similarities in several observations between NTA and the anti-DNP antibodies, as well as the serum IgM level, there was no significant correlation between the spontaneous production of NTA and the abnormal levels of the anti-DNP antibodies and serum IgM in the backcross mice in both quantitative and qualitative analyses. We also found no significant association between these polyclonal activation of B cells and H-2 complex.