Mechanical loading is paramount in regulating both the anabolic and catabolic activities of articular cartilage chondrocytes, essential for the matrix to retain its functional integrity. We have identified thymosin beta(4) as a putative mechanically regulated gene that may mediate load-enhanced synthesis and activation of matrix metalloproteinases (MMPs) 2 and 9 in articular cartilage. The objective of this study was to confirm the mechanical regulation of thymosin beta(4) and determine its effect on cartilage chondrocyte MMP production. Thymosin beta(4) mRNA expression, analysed by quantitative PCR, revealed a significant 20-fold increase in cartilage loaded for 10 min which was still evident after 30 min of loading. Treatment of primary chondrocytes with 2 and 4 micro x ml(-1) thymosin beta(4) peptide for 4 h significantly increased pro-MMP 9 expression and activation. We postulate a functional role for load-induced thymosin beta(4) in modulating the cytoskeletal organization of articular cartilage chondrocytes to affect MMP expression.