Five kinds of 15-keto-PG delta 13-reductases (enzymes I, II, III, IV and V) were separated and purified from rat liver cytosol. Four (enzymes I,II, III and IV) out of these enzymes were homogeneous by the criterion of sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weights of enzymes I, II, III and IV were estimated to be 40,000, 25,000, 64,000 and 70,000 by the electrophoresis, and 42,000, 23,000, 66,000 and 72,000 by gel filtration on a Sephadex G-200 column, respectively. All of these enzymes exhibited the NADPH-dependent activities. In the cases of enzymes I, III and V, NADH was also effective as an electron donor, but to a lesser extent in enzymes I and III. The apparent K(m) values of enzymes I, II, III, IV and V for 15-keto-PGF2 alpha with NADPH were 276, 875, 842, 948 and 2767 nM. The enzymes had isoelectric points at 4.5, 4.9, 6.2, 6.4 and 5.4, respectively. Enzyme I exhibited the double bond reductase activities toward alpha, beta-ketoalkenes such as trans-benzylidene-acetone and trans-phenyl-1-propenylketone. Enzymes III and IV also catalyzed the double bond reduction of trans-phenyl-1-propenyl-ketone. All of these enzymes were markedly inhibited by various chemicals such as dicumarol, quercitrin, p-chloromercuri-benzoic acid, 5,5'-dithio-bis(2-nitrobenzoic acid) and so on.