In a patient with hyperargininemia, enzyme replacement therapy such as whole blood exchange transfusion or erythrocyte transfusion was performed, and its effect was confirmed in vitro as well as in vivo. The patient has been treated with the restriction of protein intake, oral administration of an essential amino acid mixture, and sodium benzoate or phenylacetic acid. With these treatments, his plasma ammonia levels were controlled. On the other hand, plasma and CSF concentrations of arginine were not so well controlled. With whole blood exchange transfusion and erythrocyte transfusion, plasma arginine concentrations and plasma ammonia levels were controlled. These effects have continued for about 3 months. Furthermore, the effect of exchanged erythrocytes on the blood arginine levels was also confirmed by in vitro experimentation that mixes arginine with erythrocytes in the medium RPMI 1640. Arginine concentration of the medium did not decrease when arginine was mixed with erythrocytes of the patient, but when arginine was mixed with erythrocytes of mother or normal controls, it decreased rapidly. In calculation, 10(7) of erythrocytes of the mother or normal controls was presumed to metabolize 4-5 nmole of arginine per day. From these results, the erythrocyte exchange transfusion is considered to be effective for the control of clinical and biochemical abnormalities in this disorder.