1. Sixteen diethylstilbestrol-treated A×C line 9935 female rats on a synthetic diet containing 0.14 per cent DL-tryptophan and 26 per cent tryptophan-free casein hydrolysate were compared to thirteen similar rats on an isocaloric synthetic diet containing 26 per cent casein.

  2. The rats on the deficient diet lost more weight and showed a smaller percentage increase in pituitary weight than the control rats.

  3. Eighty-five per cent of the rats on the control diet diet with multiple mammary gland cancers in an average of 416 days compared to 56 per cent of the rats on the deficient diet in an average of 316 days.

  4. A total of 93 mammary adenocarcinomas were observed in the rats of the control group compared to 83 in the rats on the deficient diet.

  5. Sixteen Fischer line 344 female rats on the same control diet plus 0.06 per cent 2-acetyl-aminofluorene were compared to eleven similar rats on a diet containing 0.14 per cent dl-tryptophan and 0.06 per cent 2-acetylaminofluorene.

  6. Eighty-one per cent of the rats on the control diet with 2-acetylaminofluorene had benign or malignant liver neoplasms, compared to 55 per cent of the rats on the tryptophan-deficient diet. The average latent periods were 302 and 400 days, respectively.

  7. No bladder cancers were observed in the 2-acetylaminofluorene-fed rats of either group.

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Supported in part by a grant-in-aid from the United States Public Health Service and a grant from the Damon Runyon Memorial Fund for Cancer Research, Inc.

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