Forskolin, a diterpene compound isolated from the roots of Coleus forskohlii, activates adenylate cyclase in membranes from a variety of mammalian tissues. We found that forskolin (10(-7) to 3 X 10(-5) M) caused a concentration-related inhibition of IgE-mediated release of histamine and peptide leukotriene C4 (LTC4) from human basophils and lung mast cells. There was a significant linear correlation between the per cent inhibition of histamine and LTC4 release from both cell types. However, in both systems forskolin exerted a significantly greater inhibitory effect on LTC4 release than on histamine release. The concentration-response inhibition curve was paralleled by a forskolin-induced rise in cAMP levels in human leukocyte and mast cell preparations. The relationship between the effect of forskolin and the cAMP concentration was supported by the finding that forskolin inhibited the "first stage" of antigen-induced histamine release, but not the release caused by the Ca2+ ionophore, A23187. Propranolol, a competitive beta-receptor antagonist, did not block the inhibition of mediator release or the cAMP accumulation caused by forskolin. These data suggest that forskolin modulates the release of mediators of immediate hypersensitivity reactions via the activation of adenylate cyclase in human basophils and mast cells.