C-kit⁺ and c-kit⁻ early CD45R⁺ pro-B cells generate sIgM⁺ cells. C-kit⁺ and c-kits⁻ subsets of CD45R⁺ CD43⁺ BP-1⁻CD25⁻ NK1.1⁻ sIgM⁻ cells were sorted from lymphoid-enriched BM and placed in coculture with ST2 stromal cells. Cells were harvested at day 6 and costained for surface IgM and cytoplasmic μ chains as described in Materials and Methods. Costaining is shown for cells within lymphocyte light scatter gates. Percentages of cells falling in each quadrant are given. The data are representative of either two (c-kit⁺) or three (c-kit⁻) similar independent experiments performed on pooled marrow from 5 or 7 mice.