The areas of free radical biology and oxidative stress have received intensive investigation by the biomedical community in recent years. These studies have elucidated endogenous and xenobiotic-mediated mechanisms of oxyradical production, antioxidant defense mechanisms, and deleterious consequences of oxyradical fluxes that outstrip detoxification pathways. In recent years, basic comparative studies of these phenomena in a variety of organisms, particularly aquatic animals, and the observation of oxidative stress-related responses in organisms exposed to environmental contaminants, suggest a significant potential for this area to yield useful methodologies for biomonitoring. In this paper, basic cellular mechanisms of oxyradical production, detoxification, and toxicological sequalae are reviewed. Key biochemical responses associated with oxidative stress that have potential utility as biomarkers for environmental contamination are described. These responses include adaptive responses, such as increased activities of antioxidant enzymes, and oxidant-mediated toxicities, such as perturbed tissue redox status and oxidations of proteins, lipids and nucleic acids. Collectively, results from studies to date support continued research in this area. Additional research concerning both normal physiological and environmental influences on the relevant systems, and effects arising from exposure to a variety of xenobiotics, will greatly enhance the development of practical biomarkers based upon free radical biology.