The effect of ultrasound upon the postmortem proteolytic activity of muscle fibres was investigated. As a preliminary result it was demonstrated that ultrasonication released lysosomal enzymes from liver cells while cell membranes suffered little damage. Proteolysis brought about by endogenous proteinases after 2 days of fibre storage at 4 degrees C was assessed by means of sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). Ultrasonic treatment of fibres enhanced proteolytic activity, as shown by the increased intensity of 30-kDa region degradation bands. A distinct change featuring sonicated fibres was the degradation of an 87-kDa protein and the appearance of an 83-kDa peptide. Cell damage was not very extensive, although it depended upon ultrasonication and fibre conditions.