Functionally active human cytochrome P-450IID6 was expressed in Saccharomyces cerevisiae cells transformed with yeast expression vectors containing a fragment of the 2 mu-plasmid and cytochrome P-450 cDNA. Transcription was regulated by yeast acid phosphatase PHO5 promoter or chimeric GAL10-CYC1 promoter. Induced yeast cells synthesized cytochrome P-450 that was revealed by the optical absorption of the reduced CO form. Intact yeast cells were capable to metabolize the drug sparteine forming biotransformation products usually found in human. Expression of the human cytochromes P-450 seems to be a promising route to investigate new drugs and to screen genotoxic compounds.