We investigated the role of growth factors and fibronectin on matrix metalloproteinase (MMP) expression and on migration and invasion of mouse skeletal myoblasts in vitro. None of the growth factors tested significantly affected MMP-2 activity as revealed by gelatin zymography, but both bFGF and TNF- significantly increased MMP-9 activity (10 and 30-fold, respectively). The increase in secreted MMP-9 activity with TNF- stimulation was due at least in part to an increase in MMP-9 gene transcription, as an MMP-9 promoter construct was 6-fold more active in TNF--treated myoblasts than in control myoblasts, as well as an increase in MMP-9 proteolytic activation. Fibronectin, bFGF, HGF, and TGF-1 significantly augmented migration of mouse myoblasts but PDGF-BB, IGF-I, and TNF- did not. Fibronectin and bFGF also significantly augmented invasion of myoblasts across a Matrigel barrier. Plasmin co-treatment potentiated while N-acetyl cysteine (NAC) suppressed the effects of bFGF and fibronectin on myoblast migration and invasion. Transient transfection with an MMP-9 over-expression construct had only minimal effects on myoblast migration/invasion while over-expression of either MMP-2 or MMP-1 significantly augmented migration and invasion. These observations support the hypothesis that MMP activity is a necessary component of growth factor-mediated myoblast migration but that other consequences of growth factor signaling are also necessary for migration to occur.