In the past year we have continued our studies on reconstituted opiate receptors in purified systems. Purified G-proteins and adenylate cyclase were reconstituted into liposomes from detergent solutions by dialysis in the presence of phospholipids. In the reconstituted vesicles adenylate cyclase activity is stimulated by Gs and the stimulated activity is inhibited by Gi or by the beta-gamma subunit complex of bovine transducin. Further progress has been made in our efforts to obtain useful amounts of purified opiate receptors, and in characterizing the physical and biochemical properties of G-proteins. We have identified a monoclonal antibody, directed against a defined region of the amino acid sequence of bovine transducin, which also recognizes opiate receptors from NG108-15 neuroblastoma x glimoa hybrid cells. A peptide corresponding to the epitope of the antibody inhibits preciptation of opiate receptors by the antibody. A peptide from the homologous region of the porcine brain muscarinic receptor not only blocks the antibody, but also activates a number of G-proteins both in membranes and in solutions of the purified proteins. We have thus identified the signal transmitting domain of G-protein coupled receptors.