A mechanism involving free radicals is proposed for the oxidation of methane by Methylococcus capsulatus. Removal of a hydrogen atom from methane can be effected by oxygen in the form of a hydroperoxide-iron complex. Subsequent steps in the oxidation of the methyl radical so formed to formaldehyde can be accounted for by known free radical reactions. The oxidation of substrates other than methane (e.g. dimethyl ether, carbon monoxide or ammonia) can be accommodated by variations in the initial free radical mechanism.