Tumor necrosis factor alpha (TNF alpha) stimulates granulocyte- macrophage colony-stimulating factor (GM-CSF) production in human fibroblasts and other mesenchymal cells. However, relatively little is known about agents that downregulate cytokine production in these cells. In the present report we show that dexamethasone (Dexa), a synthetic glucocorticoid, markedly reduced GM-CSF production in TNF alpha-stimulated fibroblasts at both the protein and the RNA levels. CSF activity, GM-CSF protein, and RNA levels, determined by an in vitro colony-forming assay in normal human bone marrow cells, by an enzyme immunoassay, and by Northern blotting assay, were reduced to greater than 90% of control values by Dexa (1 mumol/L). Similarly, 1,25- dihydroxyvitamin D3 [1,25(OH)2D3], a hormone with possible physiologic immunoregulatory significance, reduced GM-CSF expression in a concentration- and time-dependent manner. However, this repression was less pronounced than that of Dexa, and in part due to a decreased proliferative activity. In contrast, cyclosporine A (CsA), another immunosuppressive agent, did not alter GM-CSF expression in TNF alpha- stimulated fibroblasts. Our in vitro studies suggest that by inhibiting GM-CSF production in fibroblasts, glucocorticoids and possibly 1,25(OH)2D3, but not CsA, may attenuate TNF alpha-mediated inflammatory processes and influence the regulation of hematopoiesis.