1. The tumor-producing fraction, isolated from Chicken Tumor I by means of differential centrifugation at high speed, has been investigated as regards its power to absorb ultraviolet light. A characteristic absorption spectrum was found, with a maximum at λ2575. The absorbing power of the material in that region was largely due to the presence of nucleic acid, or of a closely related compound.

2. Inactivation of the purified tumor fraction with ultraviolet light depressed the absorbing power of the material, especially in the region of 2600–2500Å. These changes were those which nucleic acid would present under the same conditions.

3. Inactivation of the tumor agent with acid or alkali was accompanied by decomposition of the tumor nucleoprotein and passage of free nucleic acid into solution.

4. Partial or complete inactivation of the tumor agent by heat, at 50° or 65deg;C., was attended by liberation of nucleic acid of low molecular weight.

5. The parallelism between tumor-producing activity and the integrity of the tumor ribonucleoprotein suggests that the nucleoprotein may be an essential part of the active principle.

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