Since hematopoietic stromal cells play a major role in regulating the growth and differentiation of hematopoietic cells, we have studied the effects of stromal cells on the in vitro growth of highly malignant murine RAW117-H10 lymphoma cells. These cells, when grown on a preformed layer of syngeneic Balb/c macrophage/monocyte cell line, J774A.1, stop proliferating 48 h after coculture. This stromal-cell-mediated growth regulation appears to require cell-to-cell contact between stromal cells and RAW117-H10 cells, since the J774A.1 cell supernatant did not significantly inhibit the in vitro growth of RAW117-H10 cells. Because one of the cytokines produced by the macrophages is tumor necrosis factor (TNF)-alpha, which is known to inhibit certain tumor cells, we also studied the effects of various concentrations of TNF-alpha on the in vitro growth of RAW117-H10 lymphoma cells. TNF-alpha did not significantly affect the growth of these cells in vitro. However, TNF-alpha induced significant morphologic changes in these cells similar to those seen in plasma cytoid differentiation. Thus, our results indicate that the macrophage-like stromal cells regulate the in vitro growth of malignant lymphoma cells, and that direct cell surface contact between stromal and lymphoma cells is necessary for the growth regulation, and the growth inhibition is not due to TNF-alpha secreted by the macrophages. TNF-alpha is associated with differentiation of these lymphoma cells.