The biotransformation of progesterone by human P4503A4 expressed in Saccharomyces cerevisiae was studied. Cytochrome P450 was produce to a specific content of 175+/-15 pmol/mg microsomal protein. Cells and microsomal fractions containing P4503A4 efficiently converted substrate to 6 beta- and 16 alpha-hydroxy progesterone, but this was not observed for control transformants.