B lymphocyte production in murine bone marrow is negatively regulated by sex steroids and the aim of this study was to identify early hormone sensitive checkpoints. Estrogen (E2) treatment reduced cμ⁺ pre-B cells, a change that occurred concomitantly with decreased Ig gene rearrangements and rag-1 transcripts. Estrogen decreased B lineage precursors in Ig transgenic mice, demonstrating that hormonal regulation is independent of the recombination process. B lineage precursors in Bcl-2 transgenic mice were resistant to estrogen treatment, suggesting that life/death decisions are involved in hormonal regulation. A previously uncharacterized population of CD43⁻cμ⁻ B lineage precursors was identified in normal, Ig transgenic, and RAG^−/− mice after estrogen treatment, revealing that down-regulation of CD43 can occur independent of Ig heavy chain expression. These cells expressed transcripts for both tdt andbcl-2, characteristics of early B-cell precursors. BrdU incorporation analysis revealed that the mitotic activity of early B-lineage cells is reduced in hormone-treated mice. We conclude that sex steroids modulate the production of B-lineage cells by influencing the differentiation, proliferation, and survival of early B-cell precursors. These findings are informative about mechanisms of hormonal regulation, as well as the significance of some differentiation-related events.