It is likely that chrysotile fibers deposited in the lower respiratory tract become rapidly coated by components of lung lining fluid. Therefore, we have used lung lining fluid and its components as part of an in vitro model to study chrysotile stimulation of superoxide anion production by the alveolar macrophage. In terms of superoxide anion production, lung lining fluid-treated chrysotile was 50% as effective as the untreated fibers. Fractionated lung lining fluid components and pure phospholipids were tested individually for their effects on chrysotile bioactivity. Pretreatment of chrysotile with lung surfactant isolated from a 30,000g pellet of lung lining fluid decreased chrysotile-stimulated superoxide anion production by 90%. The inhibitory activity of lung surfactant was found to reside in a chloroform extract containing hydrophobic proteins and lipids. Total proteolysis of the proteins did not affect the inhibitory activity of the chloroform extract, but treatment with phospholipase C significantly decreased its inhibitory activity. The inhibitory effects of lung surfactant could be simulated with phosphatidylinositol, phosphatidylserine, and phosphatidylglycerol at concentrations equivalent to those found in lung lining fluid. These results strongly suggest that phosphatidylinositol, phosphatidylserine, and phosphatdlyglycerol in lung lining fluid can modify chrysotile bioactivity for the alveolar macrophage. Together with previous results indicating that IgG enhances asbestos bioactivity, it would appear that lung lining fluid contains components that can either inhibit or enhance the bioactivity of asbestos and that it is the relative amounts of these components that determines the overall bioactivity of the fiber.