A biotransformation mixture which contained codeine and washed cells of Pseudomonas putida M10 gave rise to a number of transformation products that are of clinical importance which included hydrocodone, dihydrocodeine and 14L-hydroxycodeine. Incubations with the same organism and codeinone gave rise to 14L-hydroxycodeinone and 14L-hydroxycodeine. Cell-free extracts and membrane fractions of P. putida M10 were shown to catalyse the 14L-hydroxylation of codeinone. In addition, the potent analgesic oxycodone was shown to be produced from 14L-hydroxycodeinone. ß