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Growth and Regression of the Rous Sarcoma as a Function of the Age of the Host^*

P. M. Freire, Estelle Bryan and F. Duran-Reynals

( Department of Microbiology, Yale University School of Medicine, New Haven, Conn.)

Regression of actively growing Rous sarcomas occurred in at least 15 per cent of 192 adult or old chickens, whereas it never occurred in 1,464 chicks 15 days old when injected.

Tumor regression was generally followed by resistance manifested either by lack of growth, or by a still higher rate of regressions following further tumor inoculations. However, two of eleven birds immune to the sarcoma developed malignant epithelial tumors of the ovary.

Irrespective of their age—in 9 per cent of chicks and 13 per cent of adult chickens—a number of hosts failed to respond to a first tumor inoculation but responded to reinoculation just as normal chickens do. This state of nonresponsiveness, therefore, is different from the efficient immune reactions linked with the age factor.

^* This investigation was aided by research grants from the Jane Coffin Childs Memorial Fund for Medical Research, the National Cancer Institute of the National Institutes of Health, U.S. Public Health Service, and the American Cancer Society upon recommendation of the Committee on Growth of the National Research Council.

On leave from the Department of Health of the State of São Paulo, Brazil. Present address: Instituto Biológico, Caixa Postal 7119, São Paulo, Brazil.

Received 12/22/52.

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