T he present study examines the effects of phytohemagglutinin stimulation of a population of human (h) PBMC enriched in lymphocytes (hPBMC) on D-glucose displaceable cytochalasin B binding sites or medium-affinity sites (M-sites) in relation to glucose transport. Previously we have shown that M-sites are glucose transporters in hPBMC (Mookerjee, B.K., et al. 1981. J. Biol. Chem. 256:1290-1300). Equilibrium exchange of 3-O-methyl D-glucose in unstimulated cells revealed two populations with fast and slow flux rates. Phytohemagglutinin stimulates flux rates by converting part of the slow flux population to the fast flux population. M-sites occur in two distinct pools, one in plasma membrane and the other in microsomal fraction. Phytohemagglutinin treatment increases the plasma membrane pool size of M-sites with a concomitant reduction in the microsomal pool size without affecting the binding affinities or the total number of M-sites/cell. Data presented in this paper demonstrate that there are two pools of glucose transporters in these cells and phytohemagglutinin stimulation induces an energy-dependent net translocation of glucose transporters from an intracellular reserve pool to the plasma membrane, which accounts for greater than 60% of the increment in glucose transport.