We have succeeded in cultivating a very malignant sarcoma outside of the body. The purpose of the experiments was to develop a general method which would permit a study of the evolution of tumor tissue under known conditions and to observe living cancer cells at every instant of their growth. We used a fowl sarcoma that Dr. Rous has propagated from generation to generation for more than a year. Through the kindness of Dr. Rous, two chickens with actively growing tumors were placed at our disposal. Four series of cultures were made with fragments of the tumor extirpated from the animals in four different operations. The cultures started to grow after a very short latent period. While normal tissues of adult dogs and of young kittens began to develop respectively about forty-eight hours and twelve hours after inoculation of the plasmatic medium, sarcomatous tissue of the chicken showed, in some