The behavior of cesium ions are of interest in physiological and biochemical studies to elucidate the general properties of alkali metal ion transport and enzyme activation. In those systems studied, Cs+ activates Na/K ATPase in the absence of K+. The Cs+ concentration required to activate Na/K ATPase and transport rates varies with the system studied. Cesium - 133 NMR studies have been carried out on suspended human erythrocytes in media containing CsCl to measure intra- and extracellular Cs+ concentration and transport rate. Results indicate that Cs - 133 NMR is a promising tool for studying the distribution and transport of ions in biolagical systms.