Lactate and H(+) are suggested to promote the sickling process in red blood cells (RBCs) containing hemoglobin S. We demonstrated greater activity of the RBC monocarboxylate cotransporter MCT-1, lower RBC deformability and impaired hematological indices in sickle cell trait (SCT) carriers compared to control subjects, suggesting an involvement of MCT-1 in hemorheological disturbances in SCT carriers.