Results of tests on oxygen transfer rates and on a series of membranes, both in saline solutions and blood are reported. A series of hematological tests was carried out during the tests with blood, in order to assess the damage caused to the blood by contact with the membrane. All the results were compared with results obtained with the same equipment using Celgard membranes. A series of measurements at high liquid flow rate was carried out both with Celgard and the MFT membrane in order to eliminate the liquid-side resistance and evaluate the permeability of the membranes. The results of oxygen transfer rate into blood, when extrapolated by the “constant slope method” showed excellent agreement with the data obtained with saline solutions, justifying the use of oxygen absorption into saline during the process of screening membranes for use in blood oxygenators.