Malignant tumor cells (HeLa strain, L strain, chicken sarcoma and human uterine cancer cells) were dispersed with trypsin and inoculated onto the chorioallantoic membrane (CAM) of embryonated chicken eggs. HeLa and L cells formed tumors on CAM. Some of factors affecting the tumor formation were age of eggs used and number of cells inoculated. In the present experiments, in which 9 to 13-day old eggs were used and each egg received 5-6×10⁵ or more cells, the rate of tumor formation was practically 100%. The tumors became visible 3 or 4 days after the inoculation of cells and increased in size during the additional 3 to 5 days. The maximum size was about 15×15×5mm. The in vitro restoration or egg passage of the original cell strains was possible. While the tumors produced by either kind of cells were of the similar macroscopic appearance, their histological architectures were different. HeLa cells in the tumors were arranged in aciniform grouping, and the acini gathered together to form larger conglomerates. The L cell tumors were characterized by diffuse infiltration of round or a littleelongated cells without showing specific arrangement; this resembled the pattern of tumors produced by the chicken sarcoma cells which were more infiltrative and typically'sarcomatous.' Trypsinized cells from human uterine cancer tissues formed tumors when inoculated onto CAM. It was shown histologically that the tumors included acinous clusters of cells with undifferentiated characters; the type of cellulargrouping had a resemblance to the specific arrangement of HeLa cells, on one hand, and to that of the cells found in the original cancer tissues, on the other. Some concepts regarding growth of cancer cells are discussed, and a potentiality ofthe technique for studying cancer cells is suggested.