By the use of Ehrlich ascites tumor and mouse thymic cells incubated with glucose, succinate, or 2,4-dinitrophenol, the correlation of their electrophoretic mobility and metabolic activity was studied. Ehrlich ascites tumor cells showed increased mobility associated with suppressed oxygen uptake on the addition of glucose. Neuraminidase treatment of the cells suggests that none of the removable sialic acid contributes to the observed increase in cell mobility. Furthermore, such mobility change was related to neither the high glycolytic activity of the cells nor the consequent reduction in the pH of the incubation medium. A similar increased mobility associated with suppressed oxygen uptake was observed when 0.05 mm 2,4-dinitrophenol was added to the cells. However, the addition of succinate caused decreased mobility associated with accelerated oxygen uptake. It was concluded that the suppressive change in the rate of respiration would alter the negative surface charge of Ehrlich ascites tumor cells so as to increase their mobility and, conversely, its accelerative change would cause a decrease in their mobility. On the other hand, thymic cells from normal mice did not show in the presence of glucose any changes either in their mobility or in oxygen uptake, and the changes in the rate of oxygen uptake induced by the addition of succinate or 0.05 mm 2,4-dinitrophenol did not result in any significant alterations in their mobility. Apparently, thymic cells differ from Ehrlich ascites tumor cells in biochemical characteristics as well as in cell surface properties. The mechanism of the linkage between the mobility and oxygen uptake of Ehrlich ascites tumor cells is discussed.