Manganese catalysis of the oxidation of dopamine by air was studied as part of an investigation of possible manganese intoxication amongst a group of Aborigines living on manganese-rich soil on Groote Eylandt, in the Northern Territory of Australia. Manganese significantly increased the oxidation rate of dopamine, and the manganese complexes with some purines were especially efficient catalysts. An oxidation mechanism, involving a manganese(II)/(III) redox couple and a semiquinone free radical intermediate, is proposed. Stoichiometric hydrogen peroxide was produced by the oxidation, and the oxidation products of dopamine were highly toxic to the marine diatom Nitzschia closterium. Hydrogen peroxide and the superoxide radical did not oxidize dopamine at physiological pH. Some electrophilic compounds, including ascorbic acid, dehydroascorbic acid and thiamine, effectively inhibited dopamine oxidation. The Groote Eylandt Aborigines are likely to be deficient in ascorbic acid (vitamin C) and thiamine (vitamin B1), and these deficiencies, as well as their lifestyle, may predispose them to manganese intoxication.