Erythrocyte NAD(P)H2 glutathione oxidoreductase (EC 1.6.4.2.) activation coefficients (EGR AC) and D-glucose-6-phosphate: NADP 1-oxidoreductase, EC 1.1.1.49 (G-6-PD) activities were measured in 155 West African women to determine whether heterozygous G-6-PD deficiency produces false negative results in the EGR test for riboflavin deficiency. A positive correlation was found between EGR AC and G-6-PD activity demonstrating that heterozygous G-6-PD deficiency does result in abnormally depressed EGR AC values when G-6-PD activity is significantly reduced. However, heterozygous deficient genotypes with normal G-6-PD activity showed undepressed EGR AC values. These results indicate that, in its present form, the EGR test is not a suitable index of riboflavin status in subjects whose G-6-PD activity is less than 9 IU/g Hb when measured by the WHO-37 degrees assay.