Four bacterial cytochrome P450 enzyme activities are described in terms of their potential biotechnological use for steroid transformations, by engineered ''Rhodococcus'' strains, into commercially valuable compounds. Three selected rhodococcal P450s were characterized with microbiological, genetic, biochemical, spectroscopic and chromatographical methods. Two sterol C26- hydroxylases were demonstrated to be important for 3- hydroxysterol degradation. The role of the novel dextromethorphan N-demethylase in sterol metabolism was discussed. Furthermore, heterologous production of a mutated P450 enzyme, from ''Bacillus megaterium'', was shown in an engineered ''Rhodococcus'' strain. The activity of the mutated P450 was identified as 16?-hydroxylase of 4- androstene-3,17-dione, by ?H and ??C NMR. The detailed characterization of the cytochrome P450 enzymes, described in this book, provides insight into the catabolic repertoire of selected bacterial strains, necessary for engineering of steroid catabolic pathways in order to produce pharmaceuticals efficiently.