Summary.-Groups of male and female beagle dogs were given daily doses of 400 mg of various mixtures of naphthylamines for up to 109 months. Survivors were killed at 128 months. A variety of pathological conditions was diagnosed, but the only effect related to treatment was the induction of bladder neoplasms. All dogs which received pure 2-naphthylamine developed transitional-cell carcinomas of the bladder within 34 months. Two of 8 dogs receiving 6% 2-naphthylamine in 1 -naphthylamine developed early carcinoma and 2/8 dogs receiving 0.5% 2-naphthylamine in 1 -naphthylamine developed haemangioma of the bladder. Some of the dogs receiving 1 -naphthylamine (total dose 950 g) and the controls had focal cystitis or hyperplasia, but no neoplasia of the bladder.These results confirm the carcinogenicity of 2-naphthylamine to dogs. No carcinogenic effect of 1 -naphthylamine was observed, indicating that it is at least 200 times less potent as a carcinogen than 2 -naphthylamine. The incidence of bladder cancer in dogs fed mixtures of both naphthylamines explains why previous experimental and epidemiological studies of impure 1-naphthylamine have revealed carcinogenicity.

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