Adenylate cyclase can be resolved into at least two protein components, neither of which by itself catalyzes the formation of cyclic AMP with Mg-ATP as substrate. Mixture of the two reconstitutes Mg-ATP-dependent, fluoride- and Gpp(NH)p-stimulable activity. One, a heat-labile, N-ethylmaleimide-sensitive protein of molecular weight 190,000 can catalyze cyclic AMP formation with Mn-ATP as substrate, and is therefore proposed to be the catalytic moiety of the adenylate cyclase complex. The other protein (or proteins) is more resistant to heating or N-ethylmaleimide, and is proposed to confer upon the catalyst the ability to ultilize Mg-ATP as substrate. It is also required for the regulation of that activity by guanine nucleotides, hormones, and probably fluoride ion. The catalytic protein is found in a phenotypically adenylate cyclase-deficient (AC-) variant of S49 lymphoma cells. The thermostable regulatory protein can be resolved from the catalyst by heat treatment or N-ethylmaleimide treatment of plasma membranes of wild-type S49 cells, rat or rabbit liver, or avian erythrocytes, and is also found in a phenotypically adenylate cyclase-deficient hepatoma cell line. Mixture of AC- S49 membranes, which contain the beta-adrenergic receptor, with a crude detergent-solubilized preparation of the regulatory protein reconstitutes hormone-stimulable adenylate cyclase activity. Binding of the regulatory protein to the membranes is a time- and temperature-dependent process that requires an activating ligand of the adenylate cyclase system [fluoride, Gpp(NH)p].