After cultivation of Streptomyces hydrogenans in the presence of different steroids the activity of both 3a,20/3-hydroxysteroid dehydrogenase and 3&17/3-hydroxysteroid dehydrogenase was determined in the cell homogenate of the microorganism. By comparing the efficacy of the steroids to increase enzyme activities, steroids could be divided into 3 groups: a) steroids which stimulated preferentiallly the activity of 3a,20/3-hydroxysteroid dehydrogenase (e. g., corticosterone), b) steroids which stimulated preferentially 3p,17p-hydroxysteroid dehydrogenase (estradiol-17/3), and c) those behaving intermediately ( e . g., progesterone, 5a-dihydrotestosterone). Highest 38, l7p-hydroxysteroid dehydrogenase activity could be measured 2 h after addition of Sa-dihydrotestosterone t o the culture medium. The activity of 3a,20/3-hydroxysteroid dehydrogenase, however, increased continuously up to 4 h. 3a,20/3-hydroxysteroid dehydrogenase and 38,17,%hydroxysteroid dehydrogenase syntheses seemed to be controlled by steroids in a non-coordinate manner.In 1956 F. LINDNER isolated a strain of Streptomyces which was shown to be able to reduce 20-ketosteroids to the corresponding 20/l-hydroxysteroid (LINDNER et al. 1958). This microorganism which belongs to the order of Actinomycetales was called Streptomyces hydrogenans (ATCC 19631). NESEMANN et al. (1960) reported later that the activity of 20j3-hydroxysteroid dehydrogenase (EC I .1.1.53) which is responsible for the hydrogenation in vivo can be Stimulated if the cells were grown in the presence of certain steroids. The efficacy of the steroids to stimulate the activity of 2Oj3-hydroxysteroid dehydrogenase depends both on the structure and on the concentration of the steroid in the culture medium (NESEMANN et al. 1962).Since those early investigations, various findings on steroid metabolism catalyzed by Streptomyces hydrogenans both in vivo and in vitro were published (WACKER et al. , TRAGER and WACKER 1970, PALMOWSKI and TRAGER 1974, MARKERT et al. 1975. Besides 3 4 20j3-hydroxysteroid dehydrogenasemeanwhile a well-known and best investigated bacterial enzyme (see e. g., RLOMQUIST 1973, GIBB and JEFFERY 1973, WHITE and JEFFERY 1974, SWEET and SAMANT 1980) -previous results on steroid metabolism let suggest that the microorganism contains further steroidtransforming enzymes, e. g. A5-3j3-hydroxysteroid dehydrogenase (KOHLER and TRA-CER 1975), 3~,17~-hydroxysteroid dehydrogenase (EC 1.1.1.63) (MARKERT and TRACER 1975, DUCIIMANN and TRACER 1979a), 4-ene-3-oxosteroid-5n-reductase (EC 1.3.99.5) and d4-d5-ketosteroid isomerase (EC 5.3.3.1) (TINSCHERT and TRACER 1977).