The aim of this work was to investigate the effect of the poly-strain probiotic on oxidative-antioxidant balance and the level of matrix metalloproteinases (MMPs) in rat knee cartilage under experimental osteoarthritis. Osteoarthritis was induced by a single injection of monoiodoacetate into the knee joint of rats. Probiotic was administered daily for 14 days. Knee cartilages homogenate was used to evaluate the content of reactive oxygen species (superoxide anion and hydrogen peroxide), products of lipids peroxidation (diene conjugates, TBA-active compound, Shiff bases), to determine superoxide dismutase and catalase activity and activity of glutathione-dependent antioxidant enzymes, the level of reduced and oxidized glutathione. The level of MMPs -1, -2, -3, -8 expression was estimated by ELISA. Osteoarthritis was found to cause a significant increase in the reactive oxygen species level, lipid peroxidation products content, superoxide dismutase and catalase activity, level of all studied MMPs, and also depletion of glutathione-dependent antioxidant system and the decrease in the ratio between reduced and oxidized glutathione.The administration of the probiotic was followed by the tendency for the restoration of the parameters to the values of the control group. Thus, the administration of the probiotic to rats with osteoarthritis may be considered as an anti-inflammatory and antioxidant agent for further clinical trials. K e y w o r d s: osteoarthritis, probiotic, free radical processes, glutathione, matrix metalloproteinases.