Cesium (Cs), as well as potassium (K), is a member of alkaline metal elements. In the study of transporter, Cs has been used as a K channel blocker on cells. Doses of CsCl by the intraperitoneal route in mice indicated that a rapid transfer of Cs from blood to tissue compartments (Pinsky and Bose, 1984). Distribution of Cs in the whole bodies had been investigated. However, it is not clear that how the Cs is transported through which kinds of way at the molecular levels, and that the effect of Cs on the cell metabolisms. We examined the proliferation of HeLa cells cultured by Dulbecco’s modified Eagle medium supplemented with 10% fatal bovine serum and 10 mM of different types of alkali metals such as Li, Na, K, Rb, and Cs at 37 degree C, 5% CO2. Among them, only Cs inhibited the proliferation of the cells. The proliferation decrease is dependent on Cs concentration. Microscopic examination of the Cs-treated cells, cell membrane was not smoothly. Two different methods of live and dead assay were performed (i.e., lactose dehydrogenase-release assay and cytometry of Calcein-propidium iodide stained cells). In the Cs concentration that showed inhibition of cell proliferation, the HeLa cell membrane was not damaged by Cs. To confirm extracellular Cs uptake into the cell, intracellular cations were analyzed by capillary electrophoresis (P/ACETM MDQ). The intracellular cation content was analyzed as magnesium-based intracellular cation ratio. The intracellular Cs was detected in Cs-treated cells but not in control cell. The intracellular pH was measured by use of the pH-sensitive dye indicator BCECF. Alkalization of intracellular pH was occurred in the cells cultured with CsCl, but not in the cells cultured with the other alkali metals. Generally, intracellular pH of tumor cells was lower than normal cells. It is considered that Cs added to extracellular media uptake into the cells, and inhibition of cell proliferation possibly due to intracellular alkalization induced by Cs administration.