Hormone-sensitive adenylate cyclase contains a recently discovered protein component that is required for stimulation of cyclic AMP synthesis by hormones and guanine nucleotides; the component presumably couples the membrane receptor to the cyclase. We studied this protein (termed "N") in erythrocyte membranes of patients with pseudohypoparathyroidism, using assays of the protein's biochemical activity and of its susceptibility to radiolabeling in the presence of [32P]NAD and cholera toxin. By both assays, the protein's activity was reduced by 40 to 50 per cent in erythrocytes of five of 10 patients with Type I pseudohypoparathyroidism as compared with those of normal and hypoparathyroid subjects and one patient with Type II pseudohypoparathyroidism. If activity of the N protein is reduced in other tissues, this deficiency could cause the resistance of target organs in pseudohypoparathyroidism to parathyroid hormone and other hormones that work via cyclic AMP. Erythrocytes of five patients with Type I pseudohypoparathyroidism, all in one family, showed no defect in activity of the N protein; the biochemical defect of this family remains undefined.