One important aspect of oxidative stress is chemical modification of lipids, proteins and nucleic acids. Copper has been shown to be one of the agents causing oxidative stress. In muscles copper binds to Cys-374 of the actin monomer and catalyzes interchain S-S bond formation in F-actin. The aim of the present work was to study the functional consequences of actin modifications, induced by copper treatment of Mytilus edulis in vivo, on the in vitro motility parameters of isolated actin filaments from foot and adductor muscles. CuCl(2) treatment reduced the sliding velocity of actin filaments extracted from foot muscle by about 22% and increased their flexibility by 1.7 times, while had no effect on the motility and flexibility of adductor actin. Using immunoblotting techniques we found that copper ions induced carbonylation in foot but not in adductor actin. In samples of foot actin an increase in cross-linked oligomers and truncated monomers was detected. Carbonylated structures of actin and corresponding changes in its functional properties may be considered as biomarkers for environmental monitoring.