Cells of the Krebs ascites tumor of mice grow well in the body of the chick embryo until about the 17th day of incubation, when degeneration of the tumor can be seen in tissue sections and viable tumor cells begin to disappear from the internal organs of the embryo.

This death of tumor cells follows the appearance in the chick embryo of serum gamma globulins. Among these are antibodies which can agglutinate the tumor cells in vitro, and destroy their viability. These antibodies occur in the blood without the introduction of any foreign antigen. Their possible origin is discussed.

Small numbers of mouse tumor cells growing in the chick embryo are completely eliminated shortly after the time when antibodies ordinarily become detectable. When the number of cells present is larger, viable cells persist longer, and at still higher cell numbers, the embryo or chick is unable to eliminate the tumor, and is itself killed by it.

Gamma globulins of older birds injected into young chick embryos bearing growing tumor clear the embryonic organs of viable tumor cells.

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