More than 2 decades ago it was demonstrated that responses of some well-studied biochemical systems were not proportional to occupation of receptors by hormones specific for the receptors, even when the receptors were considered to be very closely coupled to the response. A plausible hypothesis devised to account for some of those results involved the formation of a ternary complex consisting of receptor Rhormone (or transmitter) ligand Land so-called regulatory or transducer protein X . The most successful experimental-theoretical study of such a system was that of DeLean et al.(1980) who showed that the adenylate cyclase-linked β-adrenergic system of frog erythrocyte membranes conformed closely to the theoretical model of Jacobs & Cuatrecasas (1976). Not only did conformity to the theory require the formation of ternary complex X ̄ but also of significant quantities of binary complex X ̄ X ̄ as well. Black & Leff (1983) were the first to adapt the ternary complex hypothesis to describe experiments of the types commonly carried out by pharmacologists. It was later utilized for this purpose by others such as Mackay (1987, 1988), Morgan et al.(1988), Jenkinson (1989) and Leff & Harper (1989). Although all of these writers gave due attention to the ternary complex in their formulations, they omitted from their calculations the X ̄ complex to whose significance DeLean et al.had drawn attention. In the following treatment we have reconsidered the role of the binary X ̄ complex from new points of view and have shown that R X ̄ complex contributed to formation of the ternary complex in the experiments of DeLean et al.but that agonist-receptor complex did not do so. These results contradict the suggestion of DeLean et al.concerning the “simplest” mechanistic explanation of their observations.