As is known, Curvularia lunata fungus is capable of introducing a hydroxy group at the 11 position of steroidal molecules [1] or at the 14 position of compounds of the androstane series [2, 3]. The process of 11-hydroxylation of pregnanes (for example, of cortexolone and its derivatives), which is widely used in the commercial synthesis of corticosteroid drugs, has been studied in sufficient detail. In particular, it is known that the target product yield may be accompanied by the formation of up to 10% of a 14-hydroxy compound [1]. In contrast to the hydroxylation of pregnanes, the analogous transformation of compounds of the androstane series by Curvularia lunata fungus has been studied to a much smaller extent, although development of a method for obtaining 14-hydroxy androstane derivatives is of practical interest for the synthesis of highly effective steroidal preparations [3]. Below we present the results of investigation of the hydroxylation of androst-4-ene-3,17-dione (AD, I) with the aid of Curvularia lunata fungus. The purpose of this study was to select an optimum transformation regime, isolate and identify the possible side products, and determine the conditions of formation of these products. The AD transformation was studied using a Curvularia lunata fungal culture from the collection of the Bioengineering Center (Russian Academy of Sciences, Moscow). The process was performed using mycelium washed from a growth medium. The study of the AD hydroxylation by C. lunata showed that the process is nonselective. In addition to the target product (14-hydroxyandrost-4-ene-3,17-dione, II), the fungus yields a mixture of several other substances, from which we isolated and identified (by thin-layer silica gel chromatography) 11-hydroxyandrost-4-ene-3,17-dione (III) and 14,17-dihydroxyandrost-4-en-3-one (IV). An increase in the time of AD transformation (at a concentration of 1.5 gliter) from 40 to 66 h led to a decrease in the concentration of compound II and an increase in the amount of compound IV (Table 1), while the content of compound III in the mixture exhibited no significant variations. The change in the ratio of fermentation products II and IV with increasing process duration can be explained by the formation of IV from II in the presence of 17-ketoxide reductase. Although the process of 17-keto group reduction has been described for some other microbiological cultures [4, 5], it is interesting to note the presence of this stage in the process conducted by C. lunata.