Abstract A radioimmunoassay technique for cyclic AMP has been applied to the assay of adenylate cyclase activity in liver plasma membranes by measuring the amount of cyclic AMP produced from ATP. This procedure is highly specific and sensitive and allows the measurement of nM concentrations of cyclic AMP in the presence of mM concentrations of ATP. The basal adenylate cyclase activity is proportional to the membrane concentration in the range of 50–400 μg of membrane protein per ml. Both the basal and the glucagon-stimulated activities of the enzyme depend on the concentration of ATP, with an optimum at 0.8–1.0 mM ATP in the presence of 5 mM Mg 2+ and an apparent K m of approx. 0.4 mM. Adenylate cyclase activity is stimulated by glucagon over a range of 0.05–10 nM, and is increased 10–20-fold by maximally stimulating concentrations of the hormone. Insulin did not alter either the basal or the glucagon-stimulated activity of the enzyme under a variety of experimental conditions including various concentrations of ATP and of the hormones. Insulin did not modify the degree of inhibition of adenylate cyclase produced by Ca 2+.