The nonselective β-adrenoceptor agonist, isoprenaline (pD2; 8.8±0.2), and selective β2-adrenoceptor agonists, clenbuterol (9.2±0.4) and salbutamol (7.1±0.1), inhibited the immunoglobulin E-mediated release of histamine from human lung mast cells in a concentration-dependent manner. The β2-adrenoceptor-selective antagonist, ICI118551 (erythro-(±)-1-(7-methylindan-4-yloyl)-3-isopropylaminobutan-2-ol HCl), antagonised the isoprenaline inhibition of histamine release from human lung mast cells with high affinity (apparent pKB; 9.5±0.2), whereas high concentrations of the β1-adrenoceptor-selective antagonist, CGP20712A (2-hydroxy-5-(2-(hydroxy-3-(4((1-methyl-4-trifluoromethyl)-1-H- imidazol-2-yl)-phenoxy)-propyl)-aminoethoxyl)-benzamide), were required to reverse the isoprenaline inhibition (apparent pKB; 6.5±0.3). Radioligand binding studies using [125I]-iodocyanopindolol ([125I]CYP) were performed on membranes derived from purified mast cells (>90% purity). Binding of [125I]CYP to mast cell membranes was displaced from a single binding site with a high affinity for ICI118551 (pKi; 8.9±0.1) and low affinity for CGP20712A (pKi; 6.0±0.03), indicative of a homogeneous population of β2-adrenoceptors. In contrast, in human lung membranes, these antagonists displaced [125I]CYP from two sites indicative of a heterogeneous population of β1-adrenoceptors (20%) and β2-adrenoceptors (80%). These data indicate that the β-adrenoceptor expressed by human lung mast cells and mediating inhibition of mediator release from these cells is the β2-adrenoceptor.