A virtually complete absence of glutathione reductase activity was found in the erythrocytes of all three children (one male, two females) from a consanguineous marriage. Intermediate values were found in the erythrocytes of both parents. The enzyme activity could not be restored either by addition of FAD in vitro or by administration of riboflavin in vivo. The amount of reduced glutathione in the erythrocytes was normal in each case. Severely diminished glutathione stability during incubation with acetylphenylhydrazine was observed in the erythrocytes of the siblings, as well as intermediate stability in the parents' red cells. Clinically, this deficiency was manifested by hemolytic crises after eating fava beans in the eldest daughter (patient), and possibly by cataracts in her own and in her brother's eyes. Very low activities of glutathione reductase were also found in the leukocytes of this family: 13%-15% of normal values for the children and 64%-66% for the parents. Moreover, the same deficiency was found in the purified white blood cells of the propositus: 8% of normal values in the polymorphonuclear (PMN) cells, 4% in the lymphocytes, and 15% in the monocytes, together with 11% in the platelets. Finally, we found an abnormal oxygen consumption of the propositus' PMNs after phagocytosis of zymosan particles, suggesting that the glutathione reductase reaction was involved in the bactericidal capacity of these cells.