Abstract: The present invention provides a stabilized, NAD(P)H-dependent, soluble nitrate reductase of the assimilatory type, characterized by a molecular weight of about 90,000 D in the case of electrophoresis in the presence of sodium dodecyl sulphate and a residual activity after 3 weeks at 35.degree. C. of more than 60%, obtainable by preparing a suspension of the comminuted starting material in tris/tartaric acid buffer (pH 7-8.5), adding soluble polyethyleneimine thereto in such an amount that 10 to 20% of the nitrate reductase activity passes into the precipitation obtained, separating off the precipitate and working up the supernatant according to conventional biochemical methods of fractionation in the above-mentioned digestion buffer, dialyzing the enzyme solution obtained and lyophilizing in a zwitterionic buffer.