Generation and proliferation of early B-cell progenitors have been known to require stromal cell-derived molecules. A stromal cell line, PA6, was found to produce a soluble mediator, which was distinct from interleukin 7 (IL-7) and stem cell factor and supported the proliferation of a stromal cell-dependent pre-B-cell clone, DW34. A cDNA clone encoding this DW34 growth-stimulating factor was isolated by expression cloning. The nucleotide sequence contained a single substantial open reading frame of 267 nucleotides encoding an 89-amino acid polypeptide. The amino acid sequence of this cytokine, designated pre-B-cell growth-stimulating factor (PBSF), revealed that it is a member of intercrine alpha subfamily. Recombinant PBSF stimulated the proliferation of DW34 cells for itself and, furthermore, synergistically augmented the growth of DW34 as well as bone marrow B-cell progenitors in the presence of IL-7.