A substance is regularly present in saline extracts of the Brown-Pearce rabbit carcinoma growth which is capable of fixing complement specifically in mixture with the sera of certain rabbits bearing the tumor or in which this has recently retrogressed, as the foregoing experiments have shown. The substance was not demonstrable in extracts of the normal tissues, virus papillomas, or uterine cancers of rabbits, nor in extracts of rabbit tissues infected with certain viruses (vaccine virus, Virus III, fibroma virus). The sera of normal rabbits, of those immune to a variety of infectious diseases, including syphilis, vaccinia, fibromatosis, and Virus III, and of others with uterine cancers or virus-induced papillomas, failed to fix complement specifically in mixture with extracts containing the antigen of the Brown-Pearce tumor.

The significance of the findings will be discussed in the succeeding paper, after consideration has been given to some of the properties of the serologically active material derived from the Brown-Pearce tumor.

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