This study investigated the effect of mechanical load on human mesenchymal stem cell (hMSC) differentiation under different exogenous transforming growth factor-beta1 (TGF-beta(1)) concentrations (0, 1 or 10 ng/ml).The role of the TGF-beta signalling pathway in this process was also studied. Human MSCs were seeded into fibrin-biodegradable polyurethane scaffolds at a cell density of 5 x 10(6) cells per scaffold and stimulated using our bioreactor. One hour of surface motion superimposed on cyclic compression was applied once a day over seven consecutive days. Scaffolds were analysed for gene expression, DNA content and glycosaminoglycan amount. Addition of TGF-beta(1) in the culture medium was sufficient to induce chondrogenesis of hMSCs. Depending on the TGF-beta(1) concentration of the culture medium, mechanical load stimulated chondrogenesis of hMSCs compared to the unloaded scaffolds, with a much stronger effect on gene expression at lower TGF-beta(1) concentrations. With TGF-beta(1) absent in the culture medium, mechanical load stimulated gene transcripts and protein synthesis of TGF-beta(1) and TGF-beta(3). TGF-beta type I receptor inhibitor LY364947 blocked the up-regulation on TGF-beta(1) and TGF-beta(3) production stimulated by mechanical load, and also blocked the chondrogenesis of hMSCs. Taken together, these findings suggest that mechanical load promotes chondrogenesis of hMSCs through TGF-beta pathway by up-regulating TGF-beta gene expression and protein synthesis.