1 It has recently been reported that CGP 12177 can act as an agonist at a novel secondary site within the human beta(1)-adrenoceptor. The aim of this study was to undertake a detailed pharmacological study of the effects of CGP 12177 on the human beta(2)-adrenoceptor. 2 CGP 12177 acted as a potent partial agonist of (3)H-cyclic AMP accumulation (log EC(50)-8.90+/-0.06) and CRE-mediated reporter gene transcription (log EC(50)-9.66+/-0.04) in CHO-K1 cells expressing the human beta(2)-adrenoceptor. These CGP-induced responses were antagonized by the beta(2)-selective antagonist ICI 118551 (apparent log K(D) values of -8.84+/-0.15 and -9.51+/-0.02 for the cyclic AMP and reporter gene responses respectively). 3 CGP 12177 was also able to antagonize both cyclic AMP and reporter gene responses to more efficacious beta(2)-agonists with similar log K(D) values (e.g. -9.57+/-0.15 and -10.04+/-0.096 respectively with salbutamol as agonist). 4 (3)H-CGP 12177 binding to beta(2)-adrenoceptors in intact CHO-beta(2) cells yielded a log K(D) value of -9.84+/-0.06, but indicated that the ligand dissociates very slowly from the receptor (t(1/2) for dissociation=65 min). However, studies with a Green Fluorescent Protein (GFP)-tagged beta(2)-adrenoceptor indicated that CGP 12177 does not stimulate beta(2)-adrenoceptor internalization. 5 This study demonstrates that CGP 12177 is a high affinity partial agonist of both cAMP accumulation and CRE-mediated gene transcription at the human beta(2)-adrenoceptor. It provides no evidence that CGP 12177 can discriminate a secondary site on the beta(2)-adrenoceptor analogous to that observed for the human beta(1)-adrenoceptor. However, despite its very weak actions on cAMP accumulation, the potent agonist effects of CGP 12177 on CRE-mediated gene transcription at the human beta(2)-adrenoceptor, coupled with its long duration of action, offers a potential lead for drug development for the treatment of chronic inflammatory airway diseases.