In this study, plasmalogen-free and plasmalogen-containing phospholipid (PL) mixtures were investigated, using the Wilhelmy balance and an oscillating drop surfactometer (ODS). Plasmalogen-containing films showed very low viscosity at very low surface tension compared with plasmalogen-free films. Ten mol% plasmalogens reduced the surface tension of the surfactant-like PL film containing 70 mol% dipalmitoyl phosphatidylcholine (PtdCho) from 47.8±0.9 to 25.7±0.8 mN/m. At these surface tensions the viscosity of the plasmalogen-containing monolayer was reduced by a factor of six. The viscosity of PL films that contain plasmalogens in the range of natural surfactant (2.5 mol%) was only half as high as plasmalogen-free PL films. These results suggest that monolayers of natural surfactant containing sufficient amounts of plasmalogens remain fluid in the final phase of expiration. These properties of a surfactant monolayer can facilitate the formation of a film reservoir during expiration and the re-spread of that film material during inspiration without film fragmentation. Copyright (C) 1999 Elsevier Science Ireland Ltd.