CD45R⁺ CD43⁺ BP-1⁻CD25⁻ NK1.1⁻ sIgM⁻ cells (early CD45R⁺ pro-B cells) were sorted into c-kit⁺ and c-kit⁻ subsets and assessed for proliferation potential on ST2 stromal cells (8 to 10 days) or in semisolid agar cultures (day 6). Data shown reflect combined results from two independent experiments performed on pooled marrow from five mice. Triplicate or six replicates of semi-solid agar cultures were plated in each experiment. Frequencies of clonable precursors in bone marrow were calculated from the incidences of each subset (Table 3).