1. Samples and extracts from synovial effusions were examined for the presence of fluorescent degradation products of free-radical oxidation (peroxidation). 2. Two classes of fluorescent compounds were identified and their fluorescence and physical properties are described. The fluorescence in the aqueous methanol phase of the extracts had the characteristics of conjugated Schiff bases. 3. Changes in synovial fluid and synovial fluid extracts in vitro induced by u.v. irradiation, prolonged incubation and the enzymic generation of oxidizing free radicals, with or without admixed peroxidizing arachidonic acid, and in the absence and presence of antioxidants, suggest that the fluorescent material is derived from peroxidized polyunsaturated lipids in damaged synovial tissue.