1. The agent causing Chicken Tumor I can be separated from the other constituents of the tumor filtrate by means of high speed centrifugation. The separation was practically complete when a filtrate of average viscosity (0.018 poise) was submitted to a centrifugal field of 14,000 times gravity for 2 hours.
2. Relative purification of the agent was obtained by means of differential centrifugation and washing in Tyrode's solution or in distilled water. The washed sediment gave opalescent solutions composed of minute particles of approximately, but not exactly, the same size. The dry weight of the active material separated by high speed centrifugation was 0.0008 mg. per cc. of filtrate, or about 1 part per 2800 parts of the total filtrate.
3. The tumor-producing activity of the washed sediment was significantly greater than that of the entire original filtrate. It is suggested that the gain in tumor-producing power was effected by the removal of an inhibiting factor, known to occur normally in chicken tumor extracts.