During the last decade a considerable amount of information has been accumulated regarding the experimental production of malignant tumors in laboratory animals. Several widely different types of agents are now known to be capable of inciting malignant growth. In addition to the well known carcinogenic action of x-rays and of radioactive substances, it is now clear, from the results obtained in a number of independent laboratories, that prolonged exposure to ultraviolet light2 gives rise to malignant tumors in the exposed tissues. As to biological agents, it has been recognized for some time that certain worm parasites are responsible for the genesis of malignant tumors of the urinary bladder and of the liver in man, and of the liver in the rat. Although Rous (1911) had discovered many years ago that fowl sarcoma could be transmitted by cell-free filtrates, and although Gye (1925) had been emphasizing the significance of this approach to the cancer problem, yet “the work of Peyton Rous and his collaborators has not met with its just recognition” according to Gye and Purdy (1931). The demonstration by Rous and Beard (1934) that carcinoma may be induced in rabbits with the Shope virus has, however, resulted in a revival of interest in the question of the rôle of viruses in the genesis of tumors generally. According to Rowntree, Steinberg, Dorrance and Ciccone (1937), crude wheat germ oil, given orally, produces sarcoma in rats in a very short time.