Red cell carbonic anhydrase isoenzymes I and II have been measured by means of cellulose acetate membrane electrophoresis in patients with chronic renal failure and dependent on periodic hemodialysis; these patients showed a chronic anemia due to many factors. Carbonic anhydrase isoenzymes were higher in uremic and anemic patients than in the controls. Since hemoglobin can act as a protons acceptor from the active site of carbonic anhydrase during the hydratation of CO2 (and vice-versa), the increase of carbonic anhydrase can facilitate, via the Bohr effect, the transfer of the oxygen to the peripheral tissues. Therefore an increase of red cell carbonic anhydrase can constitute a compensating mechanism of anemic hypoxia in uremic patients.