Through co-expression of the human beta 1 and beta 2 adrenergic receptors in a single tester cell (the murine L fibroblast) and through assaying the effect of the beta 1 and beta 2 selective blockers CGP 20712A and ICI 118551 on isoproterenol-stimulated adenylyl cyclase, it is shown that the maximal stimulation achievable with a given cell density of beta 1 adrenergic receptor is less than that obtained with the same density of the beta 2 adrenergic receptor. It is concluded that the efficacy of the 2 receptors differs in that the beta 1 adrenergic receptor has a lower efficacy (or intrinsic activity) than does the beta 2 adrenergic receptor.