In the first paper, the results of experiments are presented, in which an attempt was made to develope methods of cytoloyical observation and tissue culture techniques suitable for the morphological studies on Yoshida sarcoma cells in tissue culture. In the present experiments it was established that the method of cell-suspension culture was the most suitable one for such studies. Advantages of this method of tissue culture are as follows: 1) It enables us to observe minute cell structures by phase-contrast microscopy; 2) it is easy to count cell number; 3) it is possible to evaluate cellular activity; and 4) it enables us to take out cells in culture easily and to conduct various cytological studies, such as routine staining, peroxidase reaction, supra-vital staining and carbon particle phagocytosis of these cells. Next, conditions of the medium necessary for the the survival and growth of Yoshida sarcoma cells were investigated. It was found that the suitable conditions of the medium were, pH 7.2-7.4, glucose solution at the concentration of 150 mg/dl, 44% Hank's solution, 19% chick embryo extract, 29% horse serum, 8% Yoshida sarcoma ascites (extract), 0.8 mg/ml RNA, and 0.8 γ/ml vitamin B(12). The pretreatment of the medium with Yoshida sarcoma cells eliminated the problem of lag phase, and also an addition of a small portion of crushed chick embryo tissue (containing fibroblasts) improved the cellular growth. In order to keep cells floating, roller tubes were placed in an incubator kept at 37 C. Yoshida sarcoma cells cultured in such a medium under the conditions afore-mentioned. increased in number at an early stage but later their population decreased gradually. Observation was possible for a certain period of time, the maximum being 12 days.