The effects of the cell wall skeleton of Nocardia rubra on mammary gland DNA synthesis, plasma prolactin levels, and spontaneous mammary tumorigenesis in mice were studied. Female SHN mice received s.c. injections of 100 µgN. rubra cell wall every 7 days between 2 and 12 months of age. The treatment resulted in the marked inhibition of mammary tumorigenesis; incidence was significantly lower in the experimental mice than in the controls except at 9 and 12 months of age. The age of onset of mammary tumors was significantly higher in the former than in the latter. In association with these findings, the treatment also reduced normal mammary gland DNA synthesis and prolactin levels in the circulation, both of which are primary factors for mammary tumorigenesis.

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This work was supported partly by a Grant-in-Aid for Cancer Research from the Ministry of Health and Welfare, Japan.

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