The catalytic protein of rabbit hepatic adenylate cyclase, after chromatographic separation from the GTP-binding regulatory protein (G/F) (Ross, E. M. (1981) J. Biol. Chem. 256, 1949-1953), is essentially free of endogenous phospholipids. This preparation is active in the presence of Mn2+ and is markedly stimulated by forskolin, but it is stimulated only slightly by the addition of purified G/F plus an activator (GTP gamma S or NaF). The ability of activator-liganded G/F to stimulate the activity of the resolved catalyst is increased up to 8-fold by the addition of either dimyristoylphosphatidylcholine (0.3-1.5 mM optimal concentration) or several other phosphatidylcholines. Phosphatidylcholine stabilizes the catalyst to denaturation but has little effect on its basal activity or on its stimulation by Mn2+ or forskolin. It also had no stimulating effect on the activation of G/F by GTP gamma S. These data are interpreted as showing that phosphatidylcholine promotes or is required for the stimulatory interaction of activator-liganded G/F with the catalytic protein of adenylate cyclase. Lubrol 12A9, Triton X-100, cholate, lysophosphatidylcholine, digitonin, and phosphatidylserine could not substitute for phosphatidylcholine. The detergents inhibited stimulation by liganded G/F even in the presence of phosphatidylcholine. Removal of cholate from a mixture of soluble catalytic protein and phosphatidylcholine by dialysis and sucrose density gradient centrifugation caused the binding of catalytic protein to large unilamellar vesicles. This preparation was further reconstituted with increasing amounts of G/F to yield vesicles with varied G/F:catalyst ratios and similarly varied responses to G/F-mediated activating ligands.