The recent description of a Lin -AA4.1 + CD19 +B220Lo /- B1-specified progenitor (B1P) population in adult marrow adds support for the argument that these unique B cells arise from a distinct lineage. However, the origins of B1P were not investigated and their developmental relationships to conventional B2 cells remain unclear. We now report that B1P development is IL-7Rα-dependent, and negatively regulated by Bruton tyrosine kinase. Lymphoid characteristics of B1P were further studied with recombination activating gene (RAG)-1/GFP knock-in, RAG-1/Cre reporter, and VEX transgenic mice. Our results reveal that they are heterogeneous with respect to lymphocyte affiliation. RAG-1 + early lymphoid progenitors and Lin -Sca-1 +cKit LoIL-7Rα + common lymphoid progenitors from adult marrow efficiently generated CD19 +CD45R/B220 Lo/- cells in vitro and in vivo. Moreover, early lymphoid progenitors and common lymphoid progenitors produced significant numbers of peritoneal CD11b +CD5 + B1a and CD11b +CD5 - B1b cells in vivo. Finally, 2-step transplantation experiments established a differentiation pathway between conventional lymphoid progenitors, B1P, and mature B1 lymphocytes. Thus, our findings indicate that at least some B1P can be produced in adult bone marrow from primitive B2 progenitors, and suggest a developmental relationship between the major categories of B lymphocytes.