Surface dilatational behavior of spread monolayers of the surfactant protein B (SP-B) and its mixture with dipalmitoyl phosphatidylcholine (DPPC + 0.5 mol % SP-B) were investigated on the surface of a pendant phosphate buffered saline drop. Surface pressure/area isotherms of these pulmonary surfactant components were recorded in a captive bubble device. We found that the dilatational viscosity of SP-B and DPPC/SP-B films strongly decreases in the frequency range of respiration, creating an elastic film. Temperature (20 versus 30 degreesC) influences surface rheology quantitatively but not qualitatively. DPPC films containing either of the two hydrophobic pulmonary surfactant proteins, SP-B or SP-C, are predominantly elastic at breathing frequencies (0.04-0.2 Hz). We conclude that the hydrophobic surfactant proteins balance the surface rheological behavior of the thin film that covers the alveolar lining layer, by forming an interface of low compressibility at surface pressures below the protein squeeze-out. In the region of the protein squeeze-out, the film compressibility, however, increases. This behavior reduces the mechanical work required for respiration.