Articular diseases, such as osteoarthritis, is the clinical expression of the loss of cartilage function. COX inhibitors are widely used in the treatment of such pathologies for their beneficial effects on inflammation but often produce a negative activity on cartilage synthesis. In this study, we determined the effect of different prodelphinidins, the major compounds isolated from Ribes nigrum leaves, on the proteoglycans (PGs), type II collagen (coll. II) and prostaglandin E(2) (PGE(2)) production by differentiated human chondrocytes cultivated in long term (12 days) and in clusters as well as their inhibition potential on COX-1 and COX-2 in vitro. Gallocatechin trimer (GC-GC-GC) showed the higher stimulation of PGs and coll. II production (1 microg ml(-1)) and the synthesis of PGE(2) was significantly reduced by gallocatechin dimer (GC-GC), gallocatechin-epigallocatechin (GC-EGC) and GC-GC-GC at 10 and 100 microg ml(-1). The inhibition of PGE(2) synthesis was confirmed by the in vitro test on purified COX enzymes, showing the selectivity of prodelphinidins on COX-2. However, the prodelphinidins had no effects on COX activity in the whole blood assay. Our studies suggest that the prodelphinidins fractions from R. nigrum may be useful as an additive agent in the prevention of osteoarthritis.