Mometasone furoate (9 alpha, 21 dichloro-11 beta, 17 alpha dihydroxy-16 alpha methyl-1,4 pregnadiene-3, 20 dione-17-[2'] furoate) was an unexpectedly potent inhibitor of the in vitro production of three inflammatory cytokines, IL-1(1), IL-6, and TNF-alpha. The potency of mometasone furoate in inhibiting cytokine production was compared to that of hydrocortisone, betamethasone, dexamethasone, and beclomethasone. IL-6 and TNF-alpha were both produced by WEHI-265.1 (murine myelomonocytic leukemia) cells following stimulation by lipopolysaccharide (LPS). Twenty-four hours after stimulation by LPS, the cell-free supernatant fluids were removed. Their cytokine content was analyzed using ELISAs specific for each cytokine. IL-1 synthesis was induced in the harvested peritoneal macrophages of BALB/c mice by incubation with LPS for twenty-four hours. The IL-1 content in the cell-free supernatant fluids was determined by the thymocyte-costimulator bioassay. Using these systems, mometasone furoate was found to be the most potent steroid tested for inhibiting the production of the three cytokines. The IC50's were 0.05 nM (IL-1), 0.15 nM (IL-6), and 0.25 nM (TNF-alpha). The inhibition of the production of proinflammatory mediators by extremely low concentrations of mometasone furoate suggests that this steroid should be highly effective in various disorders.