During a 6 h incubation at 26 °C, Mucor hiemalis hydroxylated progesterone predominantly at the 14α site. Further incubation resulted in the production of several 14α-dihydroxyprogesterones and a novel microbial transformation product, 8(9), 14(15)-didehydroprogesterone (pregna-4,8[9], 14[15]-triene-3,20-dione). Azole inhibition indicates that the hydroxylases are cytochromes P-450. Mycelia transformed in the presence of the translation inhibitor cycloheximide failed to hydroxylate progesterone, whereas identical treatment with the transcription inhibitor actinomycin D, or heat-shock for 1 h at 37 °C prior to progesterone transformation, stimulated hydroxylation. 14α-Hydroxylation was stimulated if mycelia were pre-incubated with progesterone, transcription or translation inhibitors. These data are consistent with a model in which cytochrome P-450 steroid hydroxylase mRNA is stored in cells in a dormant form by a labile sequestering protein.