1. Cytochalasin B inhibits glucose transfer in human red cells. With glucose exit the inhibition is typically non-competitive, but hexose exchange is competitively inhibited. 2. At 16 degrees C the inhibitory constant for inhibition of 3-O-methyl glucose exchange is estimated at 1.1 X 10(-7) M while that for inhibition of glucose exit is 5.0 X 10(-7) M. 3. Uptake of labelled Cytochalasin B includes a saturable component which when correlated with the inhibition of exchange corresponds to a maximal binding of ca. 2.4 X 10(5) molecules per cell. 4. The kinetic parameters are compared with those for maltose (a competitive inhibitor acting on the outside only) and phloretin (an inhibitor acting both inside and out). 5. Kinetic evidence suggests that Cytochalasin B reacts with the inside of the hexose transfer system and that the anomalous inhibitory characteristics are due to the chemical asymmetry of the system. Independent evidence in support of this view is discussed.