Fenoterol, a beta 2-adrenergic agonist recently introduced to treat asthmatic disorders, inhibits antigen-induced histamine release from human basophil leukocytes and lung mast cells in a dose-dependent fashion. The dose-response inhibition curve is paralleled by a fenoterol-induced increase in the cAMP levels of human leukocyte preparations. The relationship between the effect of fenoterol and cAMP level is supported by the finding that the beta 2-adrenergic agonist only inhibits the first stage of antigen-induced histamine release and not the release caused by the Ca2+ ionophore, A23187. Propranolol, a competitive antagonist of beta 2-adrenergic receptor, blocks the inhibition of release and the cAMP accumulation caused by fenoterol. Finally, theophylline, a cAMP phosphodiesterase inhibitor, synergistically potentiates the inhibitory effect of fenoterol on histamine release and the accumulation of cAMP. These data suggest that fenoterol may modulate the in vivo release of the mediators of immediate hypersensitivity reactions via the activation of beta 2-adrenergic receptor linked to adenylate cyclase on human basophils and mast cells.