Purified hexose transport protein ("band 4.5") from human erythrocytes, reconstituted in vesicles of its endogenous lipids, displays minima in its circular dichroism (CD) spectrum at 222 and 207 nm, a pattern diagnostic for alpha-helical content of proteins. Upon addition of D-glucose, a saturable increment of +10-12% in negative ellipticity at 222 nm is observed stereospecifically and reproducibly. Addition of L-glucose had no effect on the CD spectrum of the transport protein. Addition of cytochalasin B (CB), a reversible inhibitor of hexose transport, had no effect itself on transporter CD spectra, but restored the spectrum at 222 nm to its original value when added in the presence of D-glucose. The observed D-glucose-induced increase in ordered secondary structure is proposed to result from incorporation into the membrane of a segment of the transport protein originally at a membrane-water interface.