Concerining the physiological properties possessed by the cell membrane, recently an attention has been called on various problems such as the cell recognition mechanism to begin with, and the mutual relationships among the intercellular communication mechanism, as well as the metabolism adjustment of membrane binding enzymes, aside from the compartmentation of substances. Essentially the physico-chemical properties of the membrane of cancer cell and proliferating cells are important in relation to the treatment of cancer. And attempts are being made to change the cell metabolism by artificially altering the physicochemical properties of the cell membrane. Cepharanthine, one of the biscoclaurin alkaloids, is known from olden days to possess a thanatophidia hemolytic property, but this property seems to bring about the change in the physico-chemical properties of the cell membrane, and it is interesting in the point that this substance may have a membrane modifying property. For these reasons, during the investigation on the actions of alkaloids on the cell membrane many interesting phenomena have been elucidated. This report presents the results recently obtained about the changes in physico-chemical properties of biological membrane by the treatment with cepharanthine. The obtained results were as follows. 1. Cepharanthine inhibits K(+)-release from red blood cells when these cells are treated with snake venom, phospholipase A, lysolecithine, lead acetate and hexane. But the alkaloid does not inhibit K(+)-release induced by ionophores, surface active agents and HVJ. 2. Cepharanthine inhibits the hyperpolalization of membrane potential induced by lead acetate or hexane. 3. Similar inhibitory effect of cholesterol on K(+)-release from the cells by the treatment with lead acetate is observed. 4. From these results it is suggested that the inhibitory action of cepharanthine on K(+)-release from cells is due to the action of decrease in membrane fluidity.