Glucocorticoids (GCS) inhibit the transcription of multiple activation-associated cytokine genes. By Northern blot analysis now we demonstrate that antiproliferative concentrations of dexamethasone and 6 alpha-methylprednisolone block mitogen-induced IL-2 gene expression in human peripheral blood mononuclear leukocytes in a concentration-dependent fashion. In addition, using a mitogen-induced proliferation assay of human peripheral blood mononuclear leukocytes, we show that GCS-mediated anti-proliferative effects are not blocked by rIL-1, rIL-2, rIL-3, rIL-4, rIL-5, rIL-6, rTNF-alpha, rTNF-beta, and rIFN-gamma, individually at 1 to 1000 U/ml, but are totally abrogated, in a concentration-dependent fashion, by the combination of rIL-1, rIL-6, and rIFN-gamma (25 to 50 U/ml for each cytokine). Thus, blockade of cytokine expression is the primary mechanism by which GCS inhibit mitogen-driven and alloantigen-induced T cell proliferation. The immunosuppressive effects of GCS are almost certainly exacted at the level of cytokine gene transcription.