1. A study has been made of the incorporation of S35-labeled methionine in vitro by the tissues of C3H mice. The observed relative rates are mammary tumor > pituitary > mammary tissue > skin > liver.

  2. No difference in the rate of incorporation was observed in skin preparations from C3H female mice, C3H male mice, or foster-nursed C3H females.

  3. No effect of estrone added in vitro, or of repeated injections of estrone prior to sacrifice, could be observed on the process in skin or mammary tissue.

  4. Estrone and testosterone added in vitro had no effect on protein-labeling by tumor slices.

  5. The incorporation of labeled methionine by pituitary tissue was studied. Glands taken from males were found to be significantly less active than those from females. The presence or absence of the milk factor in females had no effect on the incorporation of the label into protein.

  6. The pituitaries of the male animals are smaller than those of the other two groups. In addition, the C3H females have a gland which is 13 per cent smaller per gram body weight than that of foster-nursed females.

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This research has been supported by a grant-in-aid from the American Cancer Society upon recommendation of the Committee on Growth of the National Research Council.

Portions of this paper were presented before the American Society for Biological Chemists in Chicago, April, 1953 (12).

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