A comparison was made in six species of animal (rat, mouse, hamster, guinea-pig, marmoset and man) of five enzyme activities associated with the hindgut microflora. Marked differences were found in the caecal activities of azoreductase, beta-glucosidase, beta-glucuronidase, nitrate reductase and nitroreductase in the four rodents, with no one species exhibiting consistently higher or lower enzyme activity. None of the laboratory animals, including the marmoset, provided an approximation of the enzyme profile associated with human faecal flora. The results indicate that it may be invalid to extrapolate the results of bacterial metabolic studies between closely related species, or from animals to man.