The effects of dexamethasone, pentoxifylline, and MAb against endotoxin (HA-1A) on the release of various proinflammatory mediators, i.e. tumor necrosis factor-μ (TNF), IL-1β, IL-8, and prostaglandin2, by human leukocytes during stimulation with Haemophilus influenzae type B were studied. The results show that only monocytes, and thus neither lymphocytes nor granulocytes, release these mediators in response to H. influenzae. Dexamethasone inhibited the release of all of these mediators, whereas pentoxifylline only inhibited the release of TNF. HA-1A only reduced the release of IL-8 from adherent monocytes significantly and had no significant effect on the release of TNF, IL-β, and prostaglandin E2. In whole blood, no significant effect of HA-1A on the release of TNF, IL-β, IL-8, and prostaglandin E2 was found. In summary, the results of this study demonstrate that dexamethasone is the most potent inhibitor of the release of proinflammatory mediators by monocytes induced by H. influenzae type B.