Whilst the scientific community was celebrating the truly momentous discovery of a 'mixed function oxidase' another oxidase was quietly working behind the scenes, mopping up soft nucleophiles and, as it had undoubtedly being doing for aeons, aiding then unknown in the metabolism of xenobiotics and the protection of life forms. This enzyme, flavin mono-oxygenase, has subsequently been shown to be a major player, if not yet an equal partner with cytochrome(s) P450, in the metabolism of both endogenous biochemicals and foreign compounds that enter the human organism. This article outlines the importance of the flavin mono-oxygenases and examines their susceptibility to activity modulation by exogenous factors.