Inhalation of quartz (crystalline silica) can lead to the fibrosing lung disease, silicosis (Morgan and Seaton, 1984). Exposure to quartz alone is rare and most exposures occur through the presence of quartz in other dusts produced by activities such as mining, quarrying and sandblasting. Consequently, silicosis is often combined with mixed dust pneumoconiosis. A number of studies in iron-ore miners (Reichel et al., 1977; Moore et al., 1987) and in animals (Gross et al., 1960) have indicated that iron can protect against the effects of quartz. It is now believed that the characteristics of the surface of quartz particles determine their interaction with biological molecules and hence their toxicity. For example, the surface can generate a range of reactive oxygen species such as hydrogen peroxide (H2O2) and the hydroxyl radical (.OH), agents known to be toxic to cells and tissues (Fantone and Ward, 1984). The production of reactive, oxygen species can be measured using electron spin resonance (ESR) techniques. ; The two main chemical activities of the silica surface are those involving siloxane bridges (Si-O-Si) and silanols (SiOH) (lIer, 1979). Silanol groups can form hydrogen bonds with cell membranes, thought to be the main site of quartz toxicity. These groups can also dissociate at physiological pH giving a net negative charge on the silicate surface which can lead to adsorption, or coordination, of organic and inorganic cations, such as iron and aluminium. This binding of metals has been shown to reduce the toxicity of quartz (Nolan et al., 1981). Paradoxically, the interaction of iron with H2O2 can also produce .OH radicals, extremely potent oxidants, through the Fenton reaction: Fe2t + H202= Fe3 + .OH + OH- The H202 for this reaction can be produced at the quartz surface or be released from macrophages and neutrophils. The objective of our study was to examine whether treating quartz particles with iron, using ferrous or ferric chloride solutions, or mixing quartz with carbonyl iron particles could alter the surface activity of quartz and the toxic and pulmonary inflammatory effects of quartz.