The antigen that binds complement in the presence of sera neutralizing the Shope papilloma virus can be readily extracted from papillomas yielding infectious virus, but not from the normal skin of rabbits bearing the growths. The virus and the complement-binding antigen appear to have the same particle size, as determined by filtration, and they are thrown down together in the centrifuge. They are destroyed by the same amounts of heating and, in general, by the same changes in pH. It is possible, nevertheless, by irradiation with ultraviolet light, or by treatment with weak alkali, to render papilloma extracts non-pathogenic without diminishing their capacity to bind complement when mixed with immune serum.

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