The sera of rabbits bearing virus-induced papillomas have been found to bind complement when mixed with antigens consisting of extracts or filtrates of the growths containing the virus. The sera of normal rabbits, of those immune to other viruses (vaccinia, herpes, fibroma, myxoma), of rabbits with syphilis, or of those with papillomas consequent on tarring, did not fix complement upon admixture with the papilloma antigens.

The complement-binding antibody was present in the serum specimens in the same relative proportions as the virus-neutralizing antibody, and both were present in greatest amount in the sera of rabbits that had borne large papillomas over considerable periods of time. A few sera were come upon that neutralized small amounts of the virus yet failed to bind complement to any noteworthy degree in the tests.

The sera of cottontail rabbits fixed complement and neutralized the virus in much higher titer than the sera of domestic rabbits with comparable growths.

The implications of the findings will be discussed in a subsequent paper, after the properties of the complement-binding antigen have been scrutinized.

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