OBJECTIVE Recent investigations have demonstrated that the administration of MSC in mouse model of diseases provided beneficial effects. On the other hand, human adipose-derived MSC condition medium (ADSC-CM) is reported as containing beneficial secreted factors, but its role in muscle fibrosis has not been identified. The aim of this study was to investigate the inhibitory effects of MSC-CM in muscle fibrosis in vitro using the C2C12 murine muscle, myoblast cell line. METHODS C2C12 cells were cultured overnight in 0.1% albumin-Dulbecco's Modifies Eagle's Medium (DMEM). The cells were then pre-incubated in ADSC-CM for 20 min, treated with 2.5-10 ng/mL human TGFβ1 for 8-72 hours and analyzed using RT-qPCR, Western blot and immunofluorescent staining. RESULTS Treatment with 20% ADSC-CM for 3 days suppressed αSMA protein expression in TGFβ1 treated C2C12 cells. ADSC-CM stimulated the proliferation of C2C12 cells in a dose-dependent manner. Furthermore, TGFβ1 induced Acta2/αSMA mRNA expression which was inhibited by ADSC-CM treatment for 8 hours. Decorin, one of the dermatan sulfate proteoglycans and an endogenous inhibitor of TGFβ1, was expressed in ADSC-CM, but not in TGFβ1 pre-incubated ADSC-CM. CONCLUSIONS Our studies provide useful information for establishing anti-fibrotic mechanism(s) of ADSC-CM, thus facilitating potential application to prevent muscle fibrosis.