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.