Cell lines and clones were established from PB76-positive mouse fetal liver at day 13 and 14 of gestation, which proliferated with division times of a day in serum-substituted cultures under the stimulatory influence of adherent stromal cells and the cytokine IL-7 for periods longer than half a year. These lines expressed varying levels of the B lymphocyte lineage related markers PB76, B220, BP-1, VpreB and lambda 5, but no surface Ig or MHC class II molecules. All clones expressed PB76, VpreB and lambda 5 in a high percentage of cells, while B220 and/or BP-1 expression was low or undetectable in some. A cell line, and several clones established from it, all had kappa and lambda light chain genes in germ-line configuration. Either one or both of their H-chain-gene containing chromosomes carried a DH to JH. These pre B cell lines and clones could be induced to VH to DH and VL to JL rearrangements. This resulted in the development of varying percentages of sIg-positive surface, MHC class II negative, LPS-reactive B cells within 2-3 days, in the absence of contacts with stromal cells and/or IL-7. When injected into SCID mice, the cultured pre B cells populated the spleen of these mice to 5% with surface Ig-, MHC class II-positive LPS-reactive cells for greater than 25 weeks. The long-term in vitro proliferative capacity of these DH-JH rearranged pre B cell clones makes them major candidates for committed stem cells of the B lineage.