The ability of isolated human chondrocytes to produce active oxygen species has been investigated. The two methods for determining H2O2 and hydroxyl radicals (°OH) production were, by a fluorimetric method (production of dichlorofluo-rescein from a precursor in the presence of horseradish peroxidase and H2O2) and by a chromatographic method (measurement of ethylene production from γ-methiol-keto-butyric acid after °OH attack). Chondrocytes were tested, both with and without activation by phorbol myristate acetate (PMA: 10−6M), in the presence of Ca2+ (1 × 10−4M) and Mg2− (2 × 10−4M) or after variable periods of anoxia under nitrogen (4 to 12 h) followed by reoxygenation (with 95% O2, 5% CO2). Under these experimental conditions, the PMA-excited chondrocytes produced from 80 to 180 nmol of hydrogen peroxide per 1 × 106 cells and chondrocytes subjected to anoxia-reoxygenation produced up to 1700 nmol H2O2 per 1 × 106 cells. The hydroxyl radical production by PMA or anoxia-reoxygenation excited cells reached 600% of the production of non-excited cells and 1300% when they were subjected to successive stimulations by PMA and anoxia-reoxygenation. The possible pathological significance of these observations is discussed. The results indicate that stimulated human chondrocytes are capable of producing active oxygen species which could play a major role in joint inflammation and cartilage damage.