Im Serum der schwangeren Frau wurde eine 17jff-Hydroxysteroid-Oxydoreduktase nachgewiesen, die eine ausgeprägte Substratspezifität besitzt und nur mit phenolischen, nicht aber mit neutralen Steroiden reagiert; darüber hinaus zeigt das Enzym eine Stereospezifität The serum of pregnant women contains a 17jff-hydroxysteroid-oxidoreductase which shows a high substrate specificity and which reacts only with phenolic, but not with neutral steroids; in addition, the enzyme exhibits stereospecificity for the 17/?-hydroxy group. The 17/9-hydroxysteroid oxidoreductase oxidises [4- C]oestradiol-17)S by the enzyme, NADPH is a somewhat more efficient cofactor than NADH. The oxidation of the 17/?-hydroxyl group shows a maximum at pH 9.0, whereas the reduction of the 17-oxo group is optimal at pH 6.O. At pH 8.6, the MICHAELIS-MENTEN constant was found to be 5.7 · 10~6M. The enzyme has a temperature optimum at 45° and an activation energy of +10 Kcal/Mol. The Q 10 -values vary between 1.5 and 1.7. The equilibrum between oestradiol-170 and oestrone is reached after 7 hr; the equilibrium constant is 18 ± 3 · 10~9M. The activity of the enzyme is inhibited by SH-blocking agents. Addition of ammonium sulphate to the serum of pregnant women (last month of pregnancy) resulted in a 3-4fold purification of the 17/5-hydroxysteroid : NAD(P) oxidoreductase with an overall yield of 50%. The highest specific activity is found in the 40-60 per cent ammonium sulphate fraction. The serum enzyme and also the ammonium sulphate preparations are rapidly inactivated. The enzyme is stabilised by oestradiol-17/ft, but not by the addition of glycerol. The activity of the serum enzyme shows a 10 fold increase between the 3 rd. and 10 th. month of pregnancy; 18 hr. after delivery, all the activity has disappeared from the serum. This finding and the results of the kinetic studies suggest that the 170-hydroxysteroid: NAD(P) oxidoreductase, present in serum of pregnant women, is of placental origin and may be identical with the 17/?-hydroxysteroid oxidoreductase (oestradiol-17/9-dehydrogenase; EC 1.1.1.62) of human placenta.Aus früheren Versuchen ist bekannt, daß in den Erythro-näher zu charakterisieren; außerdem sollte festgestellt cyten des Menschen Enzyme vorhanden sind, welche werden, ob sich die Aktivität des nachgewiesenen Enzyms die gegenseitige Umwandlung von 17^-Östradiol und während des Verlaufes der Schwangerschaft ändert. Östron katalysieren (l-5); außerdem konnten unter Vei> In der vorliegenden Arbeit wird gezeigt, daß im Serum Wendung verschiedener Substrate weitere Enzyme des der Schwangeren eine 17/?-Hydroxysteroid : NAD(P)-Östrogen-Stbffwechsels in den roten Blutkörperchen Oxydoreduktase 1