Human erythrocyte carbonic anhydrase isozyme B was measured by a specific and immunological method. The levels of carbonic anhydrase B were determined in normal subjects, patients with hyperthyroidism, and patients with chronic obstructive lung disease and patients with epilepsy under treatment with acetazolamide, using the rapid assay method of single radial immunodiffusion. The levels of carbonic anhydrase B showed a decrease in hyperthyroidism, and increased in chronic obstructive lung diseases and epilepsies. Closely negative correlations were observed between carbonic anhydrase B and T3 resin sponge uptake or protein bound iodine (PBI) levels in hyperthyroidism. Simultaneously, carbonic anhydrase B-dependent esterase activity (active carbonic anhydrase B enzyme) was determined kinetically using the immunoadsorbent method in the above-mentioned patients. The results were compared with the total enzyme protein (active and inactive carbonic anhydrase B enzyme), estimated by the single radial immunodiffusion technique. In patients treated with acetazolamide, the "true" specific activity of the carbonic anhydrase B (carbonic anhydrase B-dependent esterase activity/total carbonic anhydrase B protein) decreased remarkably. In chronic obstructive lung disease, no remarkable changes were observed. On the other hand the specific activity was elevated in hyperthyroidism to 2 times higher than that of normal subjects. The clinical significance of these active enzymes is discussed.