The red cell membrane has an exceptionally high permeability for CO₂, P_CO2≈0.15 cm/s, which is two to three orders of magnitude greater than that of some epithelial membranes and similarly greater than the permeability of the red cell membrane for HCO₃^–. As shown previously, this high P_CO2 can be drastically inhibited by 10 μM 4,4′-diisothiocyanato-2,2′-stilbenedisulfonate (DIDS), indicating that membrane proteins may be involved in this high gas permeability. Here, we have studied the possible contribution of several blood group proteins to CO₂ permeation across the red cell membrane by comparing P_CO2 of red cells deficient in specific blood group proteins with that of normal red cells. While P_CO2 of normal red cells is ~0.15 cm/s and that of Fy_null and Jk_null red cells is similar, P_CO2's of Colton null (deficient in aquaporin-1) and Rh_null cells (deficient in Rh/RhAG) are both reduced to about 0.07 cm/s, i.e. to about one half. In addition, the inhibitory effect of DIDS is about half as great in Rh_null and in Colton null red cells as it is in normal red cells. We conclude that aquaporin-1 and Rh/RhAG proteins contribute substantially to the high permeability of the human red cell membrane for CO₂. Together these proteins are responsible for 50% or more of the CO₂ permeability of red cell membranes. The CO₂ pathways of both proteins can be partly inhibited by DIDS, which is why this compound very effectively reduces membrane CO₂ permeability.