Deletions of interleukin 7 (IL-7) or its receptor components permit fetal but not adult B cell development in mice. Mice deficient in IL-7 receptor α (IL-7Rα) had 1% the number of B cells of controls and 10% that of mice deficient in the common γ chain. As IL-7Rα is also a receptor for thymic stromal-derived lymphopoietin (TSLP), we assayed the ability of TSLP to support proliferation of fetal or adult precursor B cells. Only fetal-derived pro-B cells were able to respond to TSLP, although pre-B cells from both origins were TSLP-responsive. Fetal but not adult precursors generated a measurable B cell compartment in the absence of IL-7. The residual B cells found in IL-7Rα-deficient mice required fetal liver kinase 2 (Flk-2) for their development. Thus, IL-7Rα- and Flk-2-mediated signals account for the generation of almost all mouse B lymphocytes.