AbstractHyaluronic acid (HA) may exert different action depending on its degree of polymerization. Small HA fragments induce proinflammatory responses, while highly polymerized HA exerts a protective effect in inflammatory pathologies such as rheumatoid arthritis. In both cases the toll‐like receptor 4 (TLR‐4) seems to be involved in the modulation of the inflammation process. The aim of this study was to investigate the influence of short HA oligosaccharides (HA 4‐mers) and high molecular weight HA (HMWHA) in the inflammatory response in normal mouse chondrocytes. Messenger RNA and related protein levels were measured for TLR‐4, tumor necrosis factor‐alpha (TNF‐alpha), interleukin‐1beta (IL‐1beta), interleukin‐6 (IL‐6), and interleukin‐18 (IL‐18) in cells with and without the addition of HA. NF‐kB activation was also evaluated. 4‐mer HA treatment produced a significant up‐regulation of all parameters considered while HMWHA did not exert any activity in untreated cells although it was able to reduce the effects of 4‐ mers HA significantly. Specific TLR‐4 small interference RNA (siRNA) was used to confirm TLR‐4 as the target of HA action. This study suggests that HA may modulate proinflammatory cytokines via its different degree of polymerization and inflammatory action may be modulated as a result of the interaction between HA and TLR‐4. © 2012 International Union of Biochemistry and Molecular Biology, Inc.