Summary Phagocyte-derived free radicals are considered to play a role in fibre-related pathology and the components of the lung lining fluid could modify the surface of fibres. Therefore we examined the ability of long amosite asbestos and a range of man-made fibres to stimulate release of Superoxide anion from rat alveolar macrophages when they were in their native form (unopsonised) and opsonised by incubation in rat Immunoglobulin G. We also assessed the specific amount of opsonin adsorbed to each fibre type. In the uncoated form all of the fibres produced modest amounts of Superoxide release from macrophages. When they were opsonised however there was an effect on stimulation of release of Superoxide that was fibre-specific. Both MMVF21 and RCF 1 were dramatically enhanced in their ability to stimulate release and this was related to a high affinity of their surface for IgG. Code 100/475 and SiC were not substantially affected by opsonisation and this was reflected in their low affinity for IgG. Long amosite had low affinity for IgG but showed dramatic enhancement of capacity to stimulate Superoxide release. These fibre-specific differences in the effect of a coating of material that is found in the lung lining points out the problems of interpretation of in vitro data and more work on this important area is warranted.