The reported investigations showed that riboflavin in a concentration of 5.0 X 10(-5)M penetrates across erythrocyte membrane and that the intensity of this process depends on the incubation time. ATP added to the incubation medium to a concentration of 10(-6)M or glucose in a 10(-3)M concentration enhanced riboflavin transport across the membrane. In experiments with erythrocytes obtained from patients with chronic renal failure riboflavin penetration into the erythrocytes was inhibited.