Anti-DNA autoantibodies are the hallmark of systemic lupus erythematosus and the (NZBxNZW)F1 (NZB/W) murine model. To investigate potential defects in B cell tolerance, we followed the development of anti-DNA-specific B cells in 2-5-month-old mice transgenic for an unmutated muH chain in the normal C57BL/6 and in the NZB/W background. When the transgenic H chain was combined with a random kappa L chain repertoire about 60% of the antibodies bound to DNA. The analysis of the B cell repertoire in the spleen showed extensive receptor editing and a deletion of DNA reactivity in the C57BL/6 as well as in the autoimmune NZB/W background. NZB/W compared to C57BL/6 transgenic mice had a higher frequency of anti-DNA B cells among follicular B cells that were not censored by central tolerance mechanisms. Furthermore, positive selection of B cells with a recurrent rearrangement into the marginal zone compartment was more pronounced in NZB/W mice. Serum levels of transgenic IgM and of anti-DNA autoantibodies indicate a polyclonal activation of hyperactive B cells in the transgenic NZB/W mice. We propose different B cell receptor signaling thresholds for the NZB/W compared to C57BL/6 B cells. This could explain the quantitative differences in the B cell repertoire as well as the hyperactivity of B cells from NZB/W mice.