The aim of the present study was to investigate the influence of Cs on cultured human cells. We find that HeLa cell growth is suppressed by the addition of 10 mm CsCl into the culture media. In the Cs -treated cells, the intracellular Cs and K concentrations are increased and decreased, respectively. This leads to a decrease in activity of the glycolytic enzyme pyruvate kinase, which uses K as a cofactor. Cs -treated cells show an intracellular pH shift towards alkalization. Based on these results, CsCl presumably suppresses HeLa cell proliferation by inducing an intracellular cation imbalance that affects cell metabolism. Our findings may have implications for the use of Cs in cancer therapy.