The extracellular matrix (ECM) is considered a part of the myogenesis signaling mechanism. we hypothesized that insulin-like growth factor-I (IGF-I) modifies ECM during differentiation of mouse C2C12 cells. The myogenic effect of IGF-I (30 nmol/l) was manifested by increased myogenin and myosin heavy chain (MyHC) levels as well as fusion index (2.6 times over control) on the 3rd day of differentiation. IGF-I markedly augmented laminin, but not fibronectin. Cellular contents of integrin α3, α5 and β1 during 3-day differentiation increased in the presence of IGF-I. Treatment with IGF-I increased the expression of the long form of metalloprotease ADAM12 (100 kDa) in myocytes. In conclusion: i) IGF-I caused an increase of laminin, integrin α3 and β1 in C2C12 myogenic cells that can be secondary to stimulation of myogenesis; ii) IGF-I augmented integrin α5 and ADAM12 levels, suggesting a role of this growth factor in determination of the pool of reserve cells during myogenesis.