Most lineage marker–negative (Lin⁻)TdT⁺ cells from murine marrow lack CD34 but display c-kit at low density as well as IL-7Rα and Flk-2/Flt-3 receptors. Single cells with these characteristics generated CD45RA⁺CD19⁻ as well as CD19⁺ lymphocytes in culture. CD45RA⁺CD19⁻ marrow cells were resolved into three nonoverlapping subsets. One subset, lacking DX5 and Ly-6C antigens, yielded CD19⁺ cells in culture. Further analysis demonstrated CD24 on most Lin⁻TdT⁺ cells and all CD45R⁺CD19⁻DX5⁻Ly-6C⁻ cells. Mac-1/CD11b was absent from these two subsets of B lineage precursors, while IL-7Rα was retained during subsequent differentiation to a CD19⁺ and stromal cell–independent stage. These findings contrast with previous descriptions of B lymphocyte precursors and suggest a sequence of early differentiation events.