Purebred beagle dogs have been fed a diet supplemented with dl-tryptophan, 6 g/day, a dose which increased their normal daily intake of this amino acid approximately 7-fold. Marked focal hyperplasia of the transitional epithelium of the bladder was produced, accompanied by lymphocytic and macrophagic infiltration of the submucosa. These effects were uniformly observed in dogs fed this dose for from 3.5 months to almost 7 years. No correlation was observed between the severity of the effect and the period of feeding. These results suggest a possible role of tryptophan as a cocarcinogen in the induction of bladder cancer.

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