15-Hydroxyprostaglandin dehydrogenase was isolated from human term placenta up to a final purification of 380-fold. A spec. act. of 2000 mU/mg of protein was reached. The preparation was not homogeneous as judged by analytical disc electrophoresis. The enzyme could be stored in the presence of 50% glycerol and 10mM 2-mercaptoethanol without any loss of activity for at least one year. A distinct single protein band stained after discontinuous polyacrylamide gel electrophoresis was shown by enzymatic activity staining to correspond to 15-hydroxyprostaglandin dehydrogenase activity. Thus no evidence for the exitstence of isoenzymes was obtained. The protein in the final preparation steps showed neither alcohol dehydrogenase, NAD reductase, nor NADH oxidase activity, nor enzymatic conversion of prostaglandin or 15-oxoprostaglandin in the absence of NAD and NADH. No spontaneous reactions between NAD and prostaglandin or NADH and 15-oxoprostaglandin were detectable in the absence of the enzyme. Ethanol and glycerol slightly inhibited the reaction. Various buffers (Tris/HC1, potassium phosphate, HEPES, and triethanolamine) and salts (ammonium chloride, ammonium sulfate, potassium chloride, and sodium chloride) had different effects on the reaction rate. The pH profile of the reaction shows a plateau between pH 7.0 and 7.8 and a steep maximum at pH 9.5. A linear Arrhenius plot was obtained for the temperature dependence of the reaction from 20 to 37 degrees C. The molar activation enthalpy of the reaction was calculated to be 13.1 kcal/mole. The molecular weight of 15-hydroxyprostaglandin dehydrogenase was estimated to be 32000 -/+ 3000 by gel filtration on Sephadex G-150 in the presence of 10mM mercaptoethanol.