1. Thirty-nine A×C line 9935 female rats with cholesterol pellets containing 4–6 mg. of diethylstilbestrol implanted in their scapular region were distributed into three groups and placed on isocaloric synthetic rations containing 26 per cent protein with substitutions of 1.4 and 4.3 per cent of DL-tryptophane for tryptophane-free casein hydrolysate in two of the groups.

  2. All 39 rats survived the minimum latent period of 163 days, and 30, or 77 per cent, developed 161 macroscopic mammary cancers and a total of 436 independent mammary cancer foci.

  3. The largest proportion of tumors (79 macroscopic or 194 total) was observed in the rats on the diet containing 1.4 per cent DL-tryptophane and the smallest number (31 macroscopic and 109 total) in the rats on the diet containing 4.3 per cent DL-tryptophane.

  4. The average latent period of 316 ± 5.7 days for the tumors in rats on the 1.4 per cent tryptophane diet was significantly shorter than that for the tumors in rats on the control diet (363 ± 8.6 days), or the average latent period of 399 ± 7.5 days for the tumors in the rats on the high-tryptophane diets.

  5. The addition of approximately 1 per cent of dietary tryptophane appeared to enhance the development of diethylstilbestrol-induced mammary cancer, while the greater increase of 4 per cent in dietary tryptophane inhibited the formation of these tumors. Depletion in body weight or the consequent inanition may explain the latter result.

  6. The 161 macroscopic tumors included 107 papillary cyst adenocarcinomas, 44 adenocarcinomas and solid type carcinomas, 9 with varying amounts of squamous-cell cancer and 1 unclassified.

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Supported in part by a grant-in-aid from the United States Public Health Service, the American Cancer Society, Inc., and the Michigan Cancer Foundation.

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