The effects of several beta-adrenoceptor agonists on the relaxation of precontracted human bronchial rings and the inhibition of IgE-mediated histamine release from human lung mast cells (HLMC) were studied. For the relaxation of bronchial rings, isoprenaline, fenoterol and terbutaline were full agonists whereas salbutamol was a full agonist in some (two out of six) experiments and a partial agonist in the remainder. For the inhibition of histamine release, relative to isoprenaline, neither fenoterol, terbutaline nor salbutamol was a full agonist. Studies with the irreversible beta-adrenoceptor antagonist, bromoacetylalprenolol menthane, indicated that there was a larger beta-adrenoceptor reserve for the relaxation of precontracted bronchial rings than for the inhibition of histamine release from HLMC. Further studies indicated that the isoprenaline inhibition of histamine release was considerably more susceptible to desensitizing treatments than the isoprenaline relaxation of bronchial rings. Collectively, these data suggest that a larger beta-adrenoceptor reserve exists for the relaxation of smooth muscle than the inhibition of histamine release from HLMC and that differences in receptor reserve may contribute to the relative susceptibilities of the two systems to desensitization.