The enzyme system prostaglandin 15-hydroxy dehydrogenase, which catalyzes the inactivation of all biologically active prostaglandins, has been purified 1270-fold from human placenta. Kinetic studies on the enzyme have provided information on a well-organized control mechanism to avoid prostaglandin accumulation and for a fast prostaglandin degradation. 15-Ketoprostaglandin E2 and 13,14-dihydro-15-ketoprostaglandin E2 inhibit prostaglandin 15-hydroxy dehydrogenase non-competitively with respect to prostaglandin E2. The rate equation of enzyme reaction for two substrates was used for determination of the equilibrium constant and Michaelis constants of the enzyme. The following kinetic constants for prostaglandin 15-hydroxy dehydrogenase have been found. The equilibrium constant with repect to prostaglandin E2 is 18 muM, the Michaelis constant Km for prostaglandin E2 is 1 muM for NAD+ 44muM. The inhibition constants for 15-ketoprostaglandin E2 ar Ki(slope) = 70 muM, Ki(intercept) = 150 muM, and for 13,14-dihydro-15-ketoprostaglandin E2 Ki(slope) = 80 muM, and Ki(intercept) = 150 muM. The maximal velocity for the forward reaction is V1 = 0.45 mumol/min. These kinetic data exclude a random or ping-pong mechanism, and also a Theorell-Chance type as suggested by Braithwaite and Jarabak. We propose, therefore, a sequential ordered mechanism. The isoelectric point for prostaglandin 15-hydroxy dehydrogenase is at pH 5.35, judged by isoelectric focusing.