The human placenta contains a considerable amount of 1-pyrroline-5-carboxylate dehydrogenase (23 ± 6 μg/g; n = 12), about 25% of the concentration present in liver. The enzyme is the only form in placenta that oxidizes short- and medium-chain aldehydes, which facilitates its purification from this organ. It can be purified to homogeneity by successive chromatographies on DEAE-cellulose, 5'-AMP-Sepharose and Sephacryl S-300. From 500 g of tissue, about 2.1 units of enzyme can be obtained with a 12% yield. Placental 1-pyrroline-5-carboxylate dehydrogenase is a dimer of M(r)-63,000 subunits. It exhibits a pI of 6.80-6.65, and is specific for 1-pyrroline-5-carboxylate, the cyclic for of glutamate γ-semialdehyde (K(m) = 0.17 mM, k(cat.) = 870 min-1), although it also oxidizes short-chain aliphatic aldehydes such as propionaldehyde (K(m) = 24 mM, k(cat.) = 500 min-1). These properties are very close to those of the liver enzyme, indicating a strong similarity between the enzyme forms from both organs. The enzyme is highly sensitive to temperature, showing 50% inhibition after incubation for 0.8 min at 45°C or after 23 min at 25°C. It is irreversibly inhibited by disulfiram, and a molar ratio inhibitor: enzyme of 60:1 produced 50% inhibition after incubation for 10 min. A subcellular-distribution study indicates that the enzyme is located in two compartments: the mitochondria, with 60% of the total activity, and the cytosol, with 40% activity. The physiological role of the enzyme in placental amino acid metabolism is discussed.