We examined the effects of serum deprivation on myofibrillar proteolysis in chick myotubes. Myotubes were incubated with serum-free medium for 24 hours. N(tau)-methylhistidine release, as an index of myofibrillar proteolysis, as well as protease activities such as calpain, proteasome, and cathepsins (B+L and D) activities were increased by serum deprivation. These results indicate that serum deprivation induces calpain, proteasome, and cathepsins activities, resulting in an increase in myofibrillar proteolysis in chick myotubes.