The relations between the kinds of fatty acids and triacylglycerols present and the properties of lung surfactants were examined with modified lung surfactants. Palmitic acid-tripalmitoylglycerol, stearic acid-tristearoylglycerol and mixtures of fatty acids-triacylglycerols gave good surface activities with the lung surfactant, but oleic acid-trioleoylglycerol did not give good surface activity in vitro. Lung surfactants modified with palmitic acid-tripalmitoylglycerol, stearic acid-tristearoylglycerols and the mixtures restored the initial lung pressure-volume characteristics to the excised lung after these characteristics had been lost as a result of lavage in situ. Fatty acids gave better surface activities with the lung surfactant than triacylgycerols. The lung surfactants modified with palmitic acid, stearic acid and a mixture of fatty acids showed better surface activities in vitro and gave better lung pressure-volume characteristics in situ than those with oleic acid and triacylglycerols. The protein contained in the bovine lung surfactant was a lipoprotein in which the molar ratio of phopsholipids to protein was about 100 : 1. The molecular weight of the protein was 35000 and was reduced to 10000 by pretreatment with 2-mercaptoethanol after the removal of the phospholipids from the lipoprotein. This protein contained a large proportion of nonpolar amino acids. The lipoprotein showed spontaneous spreading with 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and with mixtures of DPPC and palmitic acid or tripalmitoylglycerol. The lipoprotein also enlarged the areas of the hysteresis loops of DPPC and the mixtures.