THE production of indole from tryptophan by the action of copper sulphate and 9 per cent (w/v) sodium hydroxide was reported by Herzfeld1 and confirmed by Baker, Dawes and Happold2 in an attempt to elucidate the mechanism of the tryptophanase enzyme system of Esch. coli3. Catalysis of indole production was shown to occur with copper and cobalt, but not with other metals (including iron, manganese, nickel, chromium, tin and cerium). This specific effect of copper and cobalt is detectable down to concentrations of 5 × 10-6 M, and both metals show approximately the same catalytic activity. Only indole formation was observed at that time.