The transport function of the purified monosaccharide transporter from human erythrocytes has been investigated. By a cycle of cholate solubilization and removal, the purified transporter was incorporated into phospholipid vesicles of about 300 A diameter, at a density of about one per vesicle. This distribution permitted an all-or-none assay for transport activity, in which the fraction of the intravesicular volume that rapidly equilibrated with D-glucose in the medium yielded an estimate of the moles of protein functional in transport. It was found that almost every transporter molecule capable of binding cytochalasin B was also capable of transport. The rate of transporter-catalyzed exchange of D-glucose between the medium and the vesicles at equilibrium was also measured. In this assay, the transport activity of the purified protein, expressed per mol of cytochalasin B binding of site, was about 5% of that of the transporter in the intact erythrocyte under similar conditions. These results show that the reduced transport activity of the purified transporter found in various rate assays is due to most of the molecules functioning at a lower rate, rather than a few molecules functioning at the in vivo rate.