(a) Production of carcinoyens or co-carcinogens Nitrosamines.-The production of Nnitrosamines from secondary amines and nitrate is promoted by enzymes or metabolites from a range of gut bacteria at normal gut pH values (Hawksworth and Hill, Br. J. Cancer, 1971, 25, 520). They may be implicated in the aetiology of gastric cancer (Hill, J. med. Microbiol., 1972, 5, xiv) following their formation in the urinary bladder from where they are readily absorbed (Hawksworth and Hill, unpublished results). Steroid metabolites.-A number of steroids are known to be carcinogenic (Bischoff, Adv. Lipid Res., 1969, 7, 165) and a role for bacterial metabolites of biliary steroids in human colon cancer has been postulated (Hill et al., Lancet, 1971, i, 95). In a study of the nuclear dehydrogenation of steroids we have, to date, demonstrated the aromatization of rings A and B (Goddard and Hill, unpublished results). Amino acid metabolites.-Tyrosine is metabolized by gut bacteria to a range of phenols (Bakke, Scand. J. Gastroenterol., 1969, 4, 603), many of which have been shown to be co-carcinogenic. Similarly, tryptophan is metabolized to a range of products which are then excreted in the urine together with similar products of hepatic metabolism; many of these have been implicated in bladder cancer (Bryan, Am. J. clin. Nutr., 1971, 24, 841). The synthetic carcinogen ethionine is produced by Esch. coli from methionine (Fisher and Mallette, J. gen. Physiol., 1961, 45, 1). Dialkyl hydrazines.-These veiy potent colon carcinogens may be intermediates in the bacterial reduction of diazo dyes.