Two applications of immobilized enzymes to the artificial oxygenation of blood are described. The first concerns facilitated transport of CO2 moving from the blood through a hydrophobic membrane. It is based on activation of the rate-limiting step, which is the dehydration of hydrogen carbonic ions to CO2 at the interface between the blood and the membrane. The addition of grafted carbonic anhydrase may hypothetically increase the diffusion rate 50 times. Multiplication by 2-8 of the CO2 flux across a silicone membrane was obtained experimentally. The second application is the use of a catalase-bound cellophane membrane which creates oxygen in the blood stream, starting from a concentrated solution of hydrogen peroxide. Experimentally, a flux of 0,4 cm3/sec-cm2 could be obtained without any bubbling and without any analytically detectable hydrogen peroxide in the acceptor solution. Oxygen production by catalase is controlled by the movement of H2O2 in the donor compartment. Catalase remains active for 48 hr in such a system. The technical problems linked to the presence of the stabilizer and to the protection of catalase against concentrated H2O2 were successfully solved. One constraining factor in the use of the catalase oxygenator was the attack on the hydrophilic support of the enzyme by the hydrogen peroxide solution. A system using both enzymes in the same device was also tentatively designed.