Red blood cells from control and riboflavin-deficient rats were separated into fractions of different mean age. Measurement of haemoglobin in the red cell fractions showed a progressive reduction in the number of young cells as the severity of deficiency increased and a corresponding increase in the old cells. The red cells from the deficient animals were significantly more fragile in all fractions than their comparable fractions in the control animals when exposed to a peroxide generating system. The increased fragility of the red cell in the riboflavin deficient rat may be due to the reduced level of red cell reduced glutathione since this substance is a necessary substrate for flutathione peroxidase which functions to destroy peroxides. Reduced glutathione is also necessary to convert methaemoglobin to haemoglobin and methaemoglobin levels were increased in red blood cells from deficient animals. Thus riboflavin deficiency through its control of the activity of glutathione reductase appears to influence red cell fragility in the rat but there is no evidence yet to suggest that the life span of the erythrocyte is reduced.