The developmental changes in the capacity for D-glucose transport of guinea pig erythrocyte membranes were compared to alterations in the electrophoretic pattern of erythrocyte membrane components. Guinea pig erythrocytes lose their D-glucose carrier functions during development. Good correlation was observed between the loss of glucose uptake and apparent decrease of the zone 4.5 of Coomassie Blue-stained membrane proteins on electrophoresis. Reconstitution of membrane preparations in liposomes resulted in a parallel change in the D-glucose uptake and D-glucose penetration of intact erythrocytes. This suggests that the decrease of D-glucose transport capacity during development is caused by the loss of one or more protein components from the erythrocyte membranes.