Summary A soluble monooxygenase system from Bacillus megaterium , which hydroxlates fatty acids primarily in the (ω-2) position in the presence of NADPH and 0 2 , is 50% inhibited by carbon monoxide at a CO:0 2 ratio of 0.25 and 85% inhibited by 5 μM p-hydroxymercuribenzoate. The latter inhibition can be reversed by ferredoxin, cysteine or glutathione. These data are interpreted as indicating the involvement of a P-450-type cytochrome and a ferredoxin-type component in the hydroxylation. This system is not inhibited by superoxide dismutase, catalase or by moderate concentrations of various hydroxyl radical scavengers.