The role of the carboxy-terminal domain of the Glut 1 glucose transporter was investigated using an antipeptide antibody to the C-terminal part of the molecule. The study was performed in fibroblasts transfected with the cDNA coding for the human insulin receptor. These cells acutely respond to insulin for glucose transport. Using antipeptide antibodies to Glut 1 and Glut 4, we first established that these cells expressed only Glut 1. Then, to define the role of the C-terminal part of Glut 1 in glucose transport, the antibodies were loaded into the cells by electroporation. When anti-Glut 1 immunoglobulins were introduced into the cells, a 60% increase in basal deoxyglucose and 3-O-methylglucose transport was observed compared to that in cells electroporated with nonimmune immunoglobulins. The stimulatory action of the antipeptide was not due to an increase in the total amount of transporters. It was found only at low glucose concentrations, suggesting that the affinity of the transporter, rather than its maximal capacity, was changed. Finally, the effect of antibody was additive to that of insulin. The interaction between the anti-Glut 1 antibody and the carboxy-tail of the transporter seems to lead to an increase in the intrinsic activity of the transporter, suggesting that this part of the molecule could be implicated in the regulation of glucose uptake.