The effect of reaction products upon an alkaline phosphatase (orthophosphoric monoester phosphohydrolase, EC 3.1.3.1) obtained from the serum of a patient with hepatic carcinoma was studied using phenyl phosphate, p-nitrophenyl phosphate and β-glycerophosphate as substrates. Inorganic phosphate showed linear competitive inhibition with any of the substrates. p-Nitrophenol gave no inhibition, and glycerol caused activation in the hydrolysis of their respective phosphate esters. The enzyme was considerably inhibited by l-tryptophan, l-phenylalanine, l-norleucine, and l-leucine. d-Isomers were ineffective. The inhibition by these amino acids in the absence of products was linear uncompetitive, but in the presence of a constant concentration of inorganic phosphate the inhibition was linear noncompetitive. These results indicate that the compounds inhibit the reaction by forming dead-end complexes with the enzyme-phosphate complex. The latter observation is consistent with a mechanism of ordered release of reaction products, in which the alcoholic product is the first, and phosphate the second, product released.