1. Theoretically, the method of cultivating tissues outside the body offers an ideal technique for the demonstration and study of cytotoxins.

2. Mouse sarcoma, which grows vigorously in the plasma of normal rats, shows little or no activity in the plasma of rats immunized by mouse sarcoma injections.

3. Rat sarcoma, readily cultivated in the plasma of normal guinea pigs, either remains quite inactive, or presents a feeble growth in the plasma of guinea pigs previously treated with rat tissues.

4. The inhibition of growth in cultures of rat and mouse sarcomata in plasma from animals of a foreign species immunized against these tissues, is due, in all probability, to the action of cytotoxins.

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