The levels of erythrocyte carbonic anhydrase isozymes (CA-B and CA-C) were determined by a quantitative immunochemical technique in order to elucidate their clinical significances in patients with iron deficiency anemia. The specific antibodies against rabbit CA-B and CA-C were also obtained, and the levels of rabbit erythrocyte carbonic anhydrase B and C isozymes were assayed immunochemically in experimental hemolytic and bleeding anemia. Simultaneously carbonic anhydrase isozymes were synthesized in vitro using rabbit reticulocyte-lysate. 1 The levels of erythrocyte CA-B per gram hemoglobin, CA-B per ml erythrocyte and CA-C per gram hemoglobin all increased but CA-C per ml erythrocyte did not increased in iron deficiency anemia. Erythrocyte CA-B and CA-C expressed per gram hemoglobin were inversely correlated to hemoglobin levels. 2 In experimental bleeding anemia and hemolytic anemia of rabbits, erythrocyte CA-B increased with the progression of anemia and with increase in reticulocyte count. However the levels of CA-C did not increased with progression of anemia. 3 In experimental bleeding anemia, it was found that the level of erythrocyte CA-B was correlated to the esterase activity and erythrocyte zinc content. 4 Most of the rabbit erythrocyte CA-B was distributed in younger erythrocytes. 5 It was confirmed that CA isozymes were synthesized in rabbit reticulocyte-lysate. The content of CA-B synthesized in lysates was much higher than that of CA-C. 6 In iron deficiency anemia, the increase of erythrocyte CA is speculated to be compensatory mechanism, which brought about the transportation of CO2 and right shifts of O2 dissociation curve.