1. By using an optical method the kinetic parameters of hexose transport across the human erythrocyte membrane were determined for several sugars. The series of half-saturated constants is as follows: 3-deoxy-3-fluoro-d-glucose = 3-O-methyl-d-glucose <d-glucose<d-mannose <3-deoxy-d-glucose<d-galactose<l-arabinose. 2. Estimations of the dissociation energy of the 3-deoxy-3-fluoro-d-glucose–carrier and d-glucose–carrier complexes suggest that the binding of the two sugars to the transport system is equivalent. 3. Incubation of the erythrocytes with 3-deoxy-3-fluoro-d-glucose results in a small but significant release of F− anion. Cells treated in this way lose their ability to transport glucose.