The effects of m-calpain isolated from the skeletal muscle of sea bass on sarcoplasmic and myofibrillar proteins isolated from the same tissue were examined in vitro. Incubation of sarcoplasmic proteins with m-calpain resulted in only a slight decrease (0.7 kDa) in the molecular weight (MW) of a 26.5 kDa protein. Degradation of myofibrils, monitored by quantification of TCA-soluble peptides generated, resulted in the maximum amount of peptides being generated after 1 h of incubation at 25°C. Noticeable modifications in the SDS-PAGE profile of digested myofibrils were observed, including partial denaturation of myosin heavy chain and the release of tropomyosin, $69 and $27 kDa doublet bands and a few polypeptides of MW lower than 20 kDa in the soluble fraction. Examination of the degradation patterns of myofibrillar proteins using Western blotting showed that a-actinin was partially degraded, with release of native a-actinin and its fragments from myofibrils, whereas desmin was highly degraded after 2 h of digestion.