Sex hormones are presumed to contribute to sexual dimorphism in the immune system. Estrogen, in particular, has been suggested to predispose women to systemic lupus erythematosus. We report here that estradiol (E₂) can break B cell tolerance and induce a lupus-like phenotype in nonautoimmune mice transgenic for the heavy chain of a pathogenic anti-DNA antibody. E₂ treatment resulted in a rise in anti-DNA serum titers and in Ig deposition in renal glomeruli. ELISPOT analysis confirmed a significant increase in the number of high-affinity anti-DNA antibody-secreting B cells in the spleens of E₂-treated mice. Hybridomas generated from E₂-treated mice express high-affinity, unmutated anti-DNA antibodies, indicating that naïve B cells that are normally deleted or anergized are rescued from tolerance induction. Finally, immunohistochemical studies revealed increased Bcl-2 expression in splenic B cells of E₂-treated mice. These data demonstrate that estrogen interferes with tolerance induction of naïve autoreactive B cells and that the presence of these B cells in the periphery is associated with up-regulation of Bcl-2.