We have shown that about 75% of the CO₂ permeability (P_CO2 ) of the human red cell is lost upon inhibition by DIDS, an efficient inhibitor of AE1. This raised the question whether CO₂ traverses the membrane via separate pathways for HCO₃^-and for H⁺ rather than across a pathway for molecular CO₂. In the former case, a carbonic anhydrase (CA) close to the membrane would be important to allow rapid conversion of the two ions to CO₂ after they have passed the membrane. CA II is believed to be associated with the AE1-Rh complex on the cytoplasmic surface of the membrane and could play such a role. We have therefore studied the CO₂ permeability of human red cells deficient in CA II using the ¹⁸O exchange technique. We find for these cells a P_CO2 of 0.12 ± 0.06 cm/s (SD; n=13), which is slightly but not significantly lower than the P_CO2 observed for normal human RBCs (0.15 ± 0.08 cm/s). However, when the CA II-deficient cells are exposed to DIDS, P_CO2 falls to 0.0052 ± 0.0022 cm/s (n=15), which is 1/10 of the value in normal RBCs treated with DIDS (0.04 cm/s). We conclude that in the presence of DIDS a small fraction of about 0.035 cm/s out of the normal P_CO2 of

0.15 cm/s (23%) becomes detectable that is dependent on CA and may thus occur via (DIDS-insensitive) transport of HCO₃^-and H⁺. The remainder of 0.115 cm/s, or ~ 80%, is independent of CA and thus likely due to transfer of molecular CO₂ across the membrane.