Surface tension (s.t.) of lung surfactant (l.s.) was measured at 37 degrees C in a closed box, filled with completely humidified gas (mixtures) such as room air, O2, N2, or alveolar gas. The film was compressed to 20% of the original area and expanded, with cycling times of 10 to 20 sec. Switching from one gas mixture to another nearly always caused a fall of s.t., but in general only temporarily. Only a gas phase containing CO2, like alveolar gas, caused a permanent fall of s.t. and a rise after its withdrawal. This was not due to a direct influence of CO2, but rather to the induced change of hypophase pH; the reaction of s.t. at end-expansion (gamma max) decreased and that at end-compression (gamma min) disappeared with a buffered hypophase. By using buffered or unbuffered hypophases, we were able to create pH values in a wide range and relate them to s.t. and CO2 concentration. With pH 5.5, the mean values of s.t. at end-compression and end-expansion were 20.2 and 51.2 mN/m, respectively. They reached the significantly higher values of 25.3 and 55.4 mN/m with pH = 8.5. With 5.6% CO2 in the atmosphere, which is comparable to alveolar gas, gamma min and gamma max had mean values of 22.9 and 52.8 mN/m, respectively. Such values correspond with a pH in the range of 6.7 to 7.1, which is supposed to be the range of pH of the pulmonary interstitium.