Reduced expression of c-kit by precursor populations corresponds to increasing maturity of B-lineage progeny . Lin⁻c-kit^Hi, Lin⁻c-kit^Lo, and Lin⁻c-kit⁻ cells were sorted from BM and placed in stromal cell free culture containing IL-7, FL, SCF, and stromal cell conditioned medium. Cells were harvested at days 4 and 7. Costaining is shown for CD45R⁺ CD43⁺ cells within lymphocyte light scatter. The lower left quadrant contains cells in Fraction A, the lower right quadrant cells in Fraction B, and the upper right quadrant cells in Fraction C as identified by Hardy et al.9 Percentages of cells in each quadrant are shown. C-kit^Hi and c-kit^Lo cultures were initiated with equal numbers of cells, but because of low frequency, half as many cells were used to initiate c-kit⁻ cultures. Representative data are shown from one of two independent experiments using pooled marrow from 10 mice. Similar trends in maturity were observed in numerous experiments with cells harvested at time points from day 2 to 7.