AQUEOUS systems were reported earlier in which a manganese-phenothiazine auto-oxidation led to the emergence of semiquinone free-radicals of the drugs1. The mildness of the required conditions permitted the postulation that this interaction might be relevant to the pharmacological action of these agents2. However, a direct biological demonstration of this specific phenomenon might require extensive technical development. Therefore, a pilot experiment was designed to discover whether manganese offered promise as a generator of free radicals in some other system of obvious biological significance. It was assumed that manganese must abound within an anatomical locus which is rich in free radicals, if this metal is indeed a biological generator of such radicals.