We describe a fortuitous observation on two rats united in parabiosis since nearly a year.

One of the two, belonging to an oncoreceptive race, fell sick of a round cell sarcoma, of extremely malignant nature, which spread itself through various metastases to all the organism, invading especially the lungs and causing tha animal’s death.

The other, of oncorefractory race, did not fall sick, although he was united to the first by abundant vascular and lymphatic communications, and that the common abdominal cavity was full of an hemorrhagic exsudation containing numberless cells coming from the tumorous decomposition. As the question is of a parabiosis of long duration and of a natural tumor of great malignity, the observation, better than the preceding ones on grafted tumors, favours the demonstration of the importance of the individual constitution of a subject sick or not of cancer.