In patients suffering from intra or extrahepatic biliary obstruction defective absorption of liposoluble vitamins results in tocopherol deficiency. Erythrocyte membranes thus have an increased susceptibility to various oxydants. Erythrocytes from 28 infants and children with biliary atresia have been studied. The usual peroxide hemolysis test (H2O2) was compared with a new method using sodium azide (NaN3), an inhibitor of intraerythrocytic catalase, as the hemolyzing agent. A much more significant correlation was found between plasma tocopherol levels and NaN3--induced hemolysis (R. coefficient = -0.872; P = 99.9%) than tocopherol levels and peroxyde hemolysis (RC = -0.477; P = 95-99%). In severely tocopherol-deficient patients (plasma levels below 0.15 mg/dl compared to normal of 0.5 mg/dl) sodium azide hemolysis is completed by the 5th hour of incubation at 37 degrees C. After the 5th hour, hemolysis begin to appear in the control media and when measured at 20 hrs is almost complete especially in the isotonic saline solution (NaCl 0.9 g/l). When abnormal red cells are incubated in their own tocopherol-supplemented plasma a complete normalization of hemolysis due to NaN3, H2O and control media is obtained. The sodium azide hemolysis test is proposed as a new and simple means of measuring tocopherol deficiency in erythrocyte membranes.