Galactose utilization was studied in 25 riboflavin-deficient patients, before and after administration of 10 mg of ribo flavin orally, for 10–15 days and 18 apparently healthy persons. Erythrocyte glutathione reductase activity was also studied in these subjects. In riboflavin-deficient patients, the tolerance to galactose loading was normal, whereas the activity of glutathione reductase in erythrocytes was lowered significantly compared to those observed in controls. Assay of galactokinase in erythrocytes and galactose-1-phosphate uridyl transferase in whole blood from riboflavin-deficient subjects revealed no significant change compared to normals. Following riboflavin administration, glutathione reductase levels were raised significantly in riboflavin-deficient subjects, but apparently the galactose tolerance and the activities of galactose-metabolising enzymes were not influenced. The possible role of riboflavin in the metabolism of galactose is discusse