Cytochalasin B, a potent inhibitor of D-glucose transport systems, binds to the glucose transporter purified from human erythrocytes as described previously (Kasahara et al., 1977). The transporter, a glycoprotein, binds 9.2 .+-. 1.3 nmol of cytochalasin B/mg of protein with a dissociation constant of 0.18 .mu.M. The binding is competitively inhibited by D-glucose (Ki = 43 mM). Phloretin, diethylstibestrol, maltose, 6-O-propyl-D-galactose, propyl .beta.-D-glucopyranoside and dithiothretitol were also linear competitive inhibitors of cytochalasin B binding. The propyl sugars have been shown to inhibit transport from either the plasma or cytoplasmic side of the membrane, respectively. The binding of cytochalasin B to the isolated transporter was inhibited by both propyl sugars.