In vitro studies of K exchange in normal erythrocytes and in K-deficient cells and an evaluation of the rate of K net transport into the deficient cells was made using K42 with a specific activity of about 1 million counts/min./mg.K. Blood samples from normal human subjects and from a case of severe hypopotassemia were used. The K exchange of normal human erythrocytes in vitro increased with increasing plasma K concn. approaching a constant level at concns. above 9 meq./l. With the low K erythrocytes when the plasma K concn. was raised, the K-influx was increased by 40% of the corresponding values for normal erythrocytes. This increase corresponded quantitatively to a demonstrated net transport of K into the low-K red blood cells. The simultaneous emigration of K-ions was not reduced.