The antioxidative and/or free-radical-scavenging activities of R-(-)- and S-(+)-ibuprofen enantiomers, as well as of the drug racemate, were studied in vitro on measuring the kinetics of (uninhibited or drug-inhibited) degradation of high molecular weight hyaluronan by hydroxyl radicals. The continual flux of OH radicals at aerobic conditions was maintained by the H2O2 + Cu2+ system. The kinetics of hyaluronan degradation was monitored indirectly by capillary viscometry. Under experimental conditions, with no drug addition, the relative viscosity ([eta]rel) decreased continuously, reaching 13% of the initial [eta]rel. value in 4 h. Each drug tested exhibited a dose-dependent protective effect against hyaluronan degradation, however R-(-)-ibuprofen demonstrated a slightly greater activity than the drug S-(+)-enantiomer.