Bone marrow stromal cells appear to be key regulatory elements in hematopoiesis and lymphopoiesis. These stromal cells respond to cytokine exposure and alter their pattern of hematopoietic growth factor production, suggesting a degree of functional plasticity. We examined the effect of two cytokines, interleukin-1 (IL-1) and IL-4, on stromal cell regulation of pre-B cell generation using the bone marrow stromal cell line, S17. Neither lymphokine potentiated pre-B cell generation in the absence of stromal cells. However, addition of either 10 U/mL rIL-1 alpha or 50 U/mL rIL-4 to cultures of bone marrow cells containing S17 cells dramatically suppressed subsequent pre-B cell formation. Preculture of S17 stromal cells with either rIL-1 or rIL-4 completely abrogated their ability to support pre-B cell generation in subsequent coculture with freshly explanted bone marrow cells. Conditioned medium from IL-1- or IL-4-treated S17 cells also suppressed pre-B-cell generation in culture. Although it is not yet known which induced stromal cell factors are responsible for failure of pre-B-cell generation in treated cultures, these data do clearly demonstrate that local levels of IL-1 and IL-4 in the hematopoietic microenvironment may play a significant role in regulation of bone marrow stromal cell function. These data also demonstrate that fibroblastic stromal cells are primary target cells that respond to cytokine concentration and affect lymphopoietic cell development.