Red blood cells of 22 patients with hyperthyroidism and 4 recurrence cases were fractionated into 2 groups of young and old red cells with Ficoll 400. Carbonic anhydrase B (CA B) and C (CA C) of each young and old red cells were measured by means of single radial immunodiffusion. CA B levels of young and old red cells of untreated patients were 4.8 +/- 1.8 (mean +/- S.D) mg/g Hb and 5.6 +/- 1.8 mg/g Hb, respectively. After administration of methimazol for about one month, the CA B level of young red cells slightly increased (7.0 +/- 1.8 mg/g Hb), then the patients were submitted to subtotal thyroidectomy. CA B levels of young (12.0 +/- 1.8 mg/g Hb) and old red cells (9.4 +/- 1.8 mg/g Hb) were not normalized one month after the treatment though high levels of serum thyroid hormone rapidly decreased after the operation. A period of 2 to 6 months was further required to return to normal levels of young (15.2 +/- 1.7 mg/g Hb) and old red cells (15.1 +/- 1.6 mg/g Hb). Although almost all the investigators reported that CA C did not diminish in hyperthyroidism, CA C levels of young and old red cells were 78 per cent and 88 per cent of the normal levels, respectively.