Abstract Parathyroid extract (PTE) as well as purified parathyroid hormone (PTH) activators of adenylate cyclase in bone and kidney, produced dose-dependent decreases in the induction of alkaline phosphatase activity by 5-iodo-2′-deoxyuridine in HeLa cells. However, the combination of PTE and 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor which also inhibits the induction of alkaline phosphatase activity, in most cases produced less than additive inhibition of enzyme induction. PTE or PTH in concentrations of up to 10 times greater than that necessary to have maximal effects on the induction of alkaline phosphatase activity produced no increase in adenylate cyclase activity, nor did they increase intracellular cAMP concentrations. In addition, PTE did not potentiate the increase in cAMP concentration produced by IBMX. It thus appears that the inhibition of alkaline phosphatase activity by PTH is not mediated by cAMP.