THE production of bladder tumours by aromatic amines has been correlated with the excretion of ortho-hydroxylated metabolites (Clayson, 1962). In the case of 2-naphthylamine, Bonser, Clayson and Jull (1951) showed that the susceptibility of a species to bladder tumour induction on feeding this agent appeared to depend on the proportion of a dose metabolised to give 2-amino-1-naphthol and its conjugates. It was further shown that 2-amino-i-naphthol was highly carcinogenic upon implantation into the bladders of mice whilst the amine itself was only feebly carcinogenic under these circumstances (Bonser, Bradshaw, Clayson and Jull, 1956; Clayson, Jull and Bonser, 1958;Allen, Boyland, Dukes, Horning and Watson, 1957).The work of Allen and co-workers (1957) appeared to show that certain ortho-hydroxylated amines formed during the mammalian metabolism of tryptophan (3-hydroxykynurenine and 3-hydroxyanthranilic acid) were bladder carcinogens in mice. Clayson (1962) has criticised this work on the grounds that the results lack statistical significance due to the small numbers of animals used. The demonstration of a high level of urinary excretion of these tryptophan metabolites in patients suffering from bladder tumours (Boyland and Williams, 1956) and the observation that in Egypt (Abul-Fadl and Khalafallah, 1961) bilharzial infections are associated with both a raised level of hydroxy-anthranilic acid excretion and with an abnormally high incidence of bladder tumours seem to support the idea that these tryptophan metabolites may be an important cause of " spontaneous " bladder tumours in man.Bonser, Clayson and Jull (1951) studied the metabolism of 2-naphthylamine in a number of species of rodents but the dose levels used and the mode of administration varied from species to species and no mention was made of the age, sex or strain of animals employed. In the case of 2-naphthylamine 2-amino-inaphthol and its conjugates are not the only carcinogenic metabolites, 2-naphthylhydroxylamine and its conjugates form a second group (Boyland, Dukes and Grover, 1961) which may be intermediates in the biosynthesis of 2-amino-1-naphthol and its derivatives (Boyland, Manson and Nery, 1960). On heating with acid (i.e. under the conditions employed by Clayson (1950) for estimating 2-amino-1-naphthol and its conjugates) these hydroxylamine derivatives are converted into 2-amino-i-naphthol (Boyland, Manson and Nery, 1960) although not quantitatively. These naphthylhydroxylamines have so far only been demonstrated by paper chromatography and have not been isolated. In view of this it would appear that they are quantitatively minor metabolites and that the method