The inducibility of the key enzyme systems controlling the microbial 9 alpha-hydroxylation of 4-androstene-3,17-dione (AD) by resting Rhodococcus sp. IOC-77 cells was investigated. Consecutive induction of the 9 alpha-steroid hydroxylating activity and the accompanying product degrading Delta(1)-steroid dehydrogenating activity of the organism was observed. The possibility of achieving a complete microbial conversion of AD into 9 alpha-hydroxy-4-androstene-3,17-dione by blocking protein synthesis was demonstrated. The inducing properties of several androstane and pregnane compounds as well as the influence of some functional groups in their steroid structure were discussed.