B-lymphopoiesis decreases with age. We studied how aging affects bone marrow stromal cells, because they provide the growth factors and cell contacts required for B-lymphopoiesis. No differences were noted in the cell-surface phenotype of young and old primary-cultured stromal cells. Fluorescence-activated cell sorter-purified stromal cells from old mice were deficient in the ability to support the proliferation of interleukin-7 (IL-7)–specific B-lymphoid cell lines. The kinetics of this response indicated that IL-7 was not immediately available from stromal cells of either age and was further delayed on aged stromal cells. The levels of IL-7 protein within stromal cells were equivalent between young and old animals, suggesting that the production of IL-7 was not altered by aging. Negligible amounts of IL-7 were found either freely secreted or in the extracellular matrix of cultures of young and old marrow. Contact between the lymphoid cells and the primary stromal cells was required for detectable proliferation, suggesting that cell contact was required for the release of IL-7. We propose that stromal cells regulate B-lymphopoiesis by limiting the amount of IL-7 available to the developing precursors. Therefore, we conclude that the age-related decrease in the function of bone marrow stromal cells is related to the impaired release of IL-7.