1. Sixty-five female rats of the Fischer Line 344 were placed on isocaloric synthetic diets containing 0.06 per cent 2-acetylaminofluorene.

  2. The rats were divided into four groups receiving, respectively, 45 per cent casein, 26 per cent casein, 25 per cent tryptophane-free casein hydrolysate plus 1.4 per cent DL-tryptophane, and 22 per cent tryptophane-free casein hydrolysate, plus 4.3 per cent DL-tryptophane.

  3. Fifty-one rats survived for at least 100 days on the various dietary regimens and consumed a daily average of nearly 4 mg. of 2-acetylamino-fluorene.

  4. The rats on the 45 and 26 per cent casein diets maintained their body weight, while those on the 1.4 per cent and 4.3 per cent tryptophane diets lost, respectively, 20 and 40 per cent.

  5. Benign and malignant hepatomas were observed in the majority of rats in each group. The per cent of malignant hepatomas varied from 37 in the rats on the 26 per cent casein diet to 92 in the rats on the 45 per cent casein diet and was, respectively, 73 and 75 per cent in the rats on the 1.4 and 4.3 per cent tryptophane diets.

  6. Squamous carcinoma of the external auditory meatus occurred in 20 per cent of the rats on the 26 per cent casein diet and 1.4 per cent tryptophane diet.

  7. Bladder cancer occurred in 100 per cent of the rats on the 1.4 per cent tryptophane diet and in 92 per cent of the rats on the 4.3 per cent tryptophane diet. No bladder cancers were observed in the rats on the 45 per cent or 26 per cent casein diets.

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Supported in part by a grant-in-aid from the National Cancer Institute, Public Health Service.

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