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Studies on the Mouse Mammary Tumor Agent^*

II. The Neutralization of the Agent by Placenta

(From the Roscoe B. Jackson Memorial Laboratory, Bar Harbor, Maine)

A filtrate of spontaneous mammary tumor induced tumors in 36 per cent of the inoculated animals. The filtrate, mixed and incubated with minced liver, induced tumors in 16.6 per cent of inoculated animals. This is interpreted as due to the autolytic processes going on in the minced tissue, and evidence is presented that no great inhibition of the agent takes place in the livers of living animals. The filtrate mixed with minced placentas induced tumors in 3.5 per cent of the inoculated animals. Some of this decrease in activity is probably due to autolysis; but, as blood in females in late pregnancy apparently carries none of the agent, the evidence seems to indicate that the placenta actively destroys or inhibits the agent.

^* Assisted by grants from the Jane Coffin Childs Memorial Fund for Medical Research, the American Cancer Society on the recommendation of the Committee on Growth of the National Research Council, and the National Cancer Institute of the National Institute of Health, United States Public Health Service.