Red-cell conversion rate of pyridoxine to pyridoxal phosphate, and globin synthesis were measured before and after oral riboflavin in a patient with heterozygous beta-thalasaemia in 3 separate trials. In this patient a very slow B6 conversion rate increased to normal on each occasion after riboflavin, and there was a marked increase in synthesis of alpha and beta globin chains but no change in beta/alpha ratio. This was confirmed in a similar patient after a single trial of oral riboflavin. In 5 control subjects after a single trial of riboflavin the red-cell pyridoxine conversion rate increased whether the initial rate was fast or slow, and there was a considerable increase in globin synthesis in 3 of these.