In 18 beta-thalassaemia families from the Ferrara area the incidence of an inherited low flavin mononucleotide (FMN)-dependent pyridoxine phosphate (PNP) oxidase activity, a sensitive indicator of red-cell FMN deficiency, is higher in related members in these families than in the unrelated spouses and controls subjects without family history of thalassaemia. This suggests slower red-cell riboflavin metabolism in thalassaemia families, which may have resulted from selection in combination with thalassaemia by malaria. However, there was a markedly higher incidence of red-cell flavin adenine dinucleotide (FAD) deficiency in thalassaemia heterozygotes than in their normal relatives. This was indicated by higher stimulation of FAD-dependent glutathione reductase (GR) activity by FAD and lower GR activity per red cell, and suggests a marked additive effect by thalassaemia on the red cell FAD deficiency that results from the inherited slow riboflavin metabolism. There is evidence that diversion of FAD to other FAD-dependent enzymes might be an important factor.