Intact human red blood cells can synthesize FAD and FMN from riboflavin. The rate of synthesis of FAD is linearly proportional to the concentration of riboflavin in the medium at levels below 0.9 µM. With 0.9 µM riboflavin, the rate of synthesis is about 0.1 mµmole FAD/ml. red blood cells/hour. Incubation of red blood cells with riboflavin can result in increased red cell glutathione reductase activity when the enzyme is measured in the absence of added FAD. This indicates that the FAD concentration in the red cells increased during the incubation. The rate of incorporation of radioactive riboflavin into red blood cells is the same whether the cells are suspended in plasma or in a phosphate-saline-glucose medium. The time it takes for half the FAD in normal human red blood cells to turn over is calculated to be about 6 days, assuming a single mixing pool of red cell FAD.