We have confirmed that phytohemagglutinin (PHA) rapidly enhances the uptake of potassium (K+) by human blood lymphocytes. PHA, however, did not produce an increase in lymphocyte K+ concentration. The apparent steady-state of cell K+ concentration despite the marked increase in uptake of 42K+ could be explained by either an increase in K+-K+ exchange or an increase in concentrative (active) K+ accumulation in association with an increase in the leak of K+ from the cell. We compared, therefore, the uptake of 42K+ with the decrement in cellular K+ content when active transport was inhibited by ouabain. These studies established that K+-K+ exchange was negligible in human blood lymphocytes and that the increase in 42K+ uptake after PHA treatment represented concentrative transport. Our studies did indicate that 42K+ exodus from PHA treated lymphocytes increased markedly from 19 to 38 mmol-1 cell water-1-h-1. Within the same time period K+ influx into PHA-treated lymphocytes increased from 20 to 38 mmol-1 cell water-1-h-1. Thus, PHA produces a marked increase in the permeability of the lymphocyte membrane to K+, and the increase in active K+ influx in PHA-treated lymphocytes may represent a homeostatic response by the membrane K+ transport system to the increase in K+ efflux. Increased K+ turnover was observed at the lowest concentrations of PHA which produced an observable increase in [3H]thymidine incorporation into DNA. Thus, PHA produces an increase in K+ permeability that closely parallels its mitogenic effect. The rapid increase in K+ influx preceding blastogenesis and mitogenesis is required, therefore, to maintain normal intracellular K+ concentration. An adequate intracellular K+ concentration is essential for the synthetic processes required for cell transformation or division.