Studies in Leukemia. XII. Interference and Reactivation Experiments with Cell-Free Tissue Extracts of Leukemic Swiss Mice

1. The virulence of the Swiss mouse leukemia agent can be enhanced by rapid brain to brain passages.

2. The existence of an occult agent in the tumor tissue is demonstrable by interference and reactivation experiments.

3. Tumor extracts inhibit the leukemogenic activity of brain homogenates of leukemic mice.

4. Mixtures of tumor extracts and heat-inactivated brain extracts, neither of which is capable of inducing leukemia alone, have leukemia-inducing activity.

5. A heat-labile inhibitor is probably responsible for the masking of the virus in tumor extracts.