AbstractB cell development is dependent on both direct interactions with stromal cells and their secreted cytokines. The precise mechanisms by which these interactions regulate B cell differentiation are currently unknown. We report here that a novel growth factor thymic stromal‐derived lymphopoietin (TSLP) can replace the activity of interleukin‐7 (IL‐7) in supporting B cell developmentin vitro.TSLP was found to promote the proliferation and differentiation of committed B220+B cell progenitors from day 15 fetal liver. Phenotypic analysis of these cells revealed that they are at the pro‐B cell stage of differentiation and express cell surface markers characteristic of pro‐B cells cultured in IL‐7. TSLP can replace the activity of IL‐7 in supporting the progression of B lymphocytes from uncommitted bipotential precursors. In the absence of either TSLP or IL‐7, the progeny of cells that give rise to mature B lymphocytes fail to develop from these bipotential precursors. Moreover, TSLP can substitute for IL‐7 in supporting the sustained proliferative response exhibited by B cell progenitors from CBA/N mice. Together these results show that TSLP can replace the requirement for IL‐7 duringin vitr