Antisera prepared in rabbits and guinea pigs against mouse tissue possessing or free of the mammary tumor agent (MTA) were investigated for their neutralizing effects, in vivo and in vitro, upon the MTA.

In none of the experiments was a significant difference observed in the incidences of mammary cancer in mice which received: (a) the agent-suspension prepared with saline or distilled water, (b) the agent-suspension mixed with normal rabbit or guinea pig serum, or (c) the same agent-suspension mixed with antisera produced against normal mammary glands without the MTA.

In comparable experiments it was observed that antisera prepared against normal mammary glands with the MTA were as effective in neutralizing the agent as were sera elicited against cancerous tissues containing the MTA.

The neutralizing effects of the agent-antisera were dependent, in most tests, upon the source of the agent used to prepared the antiserum and agent-suspension.

An antigenic difference was noted between the MTA of a cancerous strain (A stock) and that of the F1 hybrids.

Evidence was obtained that the antigenic properties of the agent in a transplanted tumor may become altered with continued passage of the tumor in agent-free animals.

The interpretation of these data should await confirmation in future studies.

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Assisted by grants from the National Cancer Institute of the National Institutes of Health, Public Health Service; the American Cancer Society upon recommendation by the Committee on Growth of the National Research Council; the American Cancer Society, Minnesota Division; and the Damon Runyon Memorial Fund.

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