The injection of tumor extracts and their active protein fractions into rabbits induced the formation of precipitins and neutralizing antibodies. When the major portion of proteins in the tumor extract had been eliminated it induced the formation of neutralizing antibodies, but not of precipitins. The tumor agent, more highly purified by removal of the viscous fraction, did not induce precipitins, and only 2 out of the 15 sera gave any evidence of neutralizing bodies. After the removal of the major portion of protein, the extracts showed insufficient interaction with the sera to fix complement.

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