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.