Myofibrils isolated from bovine longissimus (L), semitendinosus (ST) and psoas major (PM) muscles at-death and at 1, 2, 3, 6 and 10 days postmortem storage (2 and 25°C) were analyzed with sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. One of the subunits of troponin, troponin T, disappeared from L and ST muscle during postmortem storage at 25°C, and concurrently a 30,000 dalton component appeared. Storage of muscles at 25°C accelerated these changes in myofibrils from L and ST muscles, but SDS polyacrylamide gels of PM muscle changed little during storage at either 2 or 25°C. Crude preparations of a Ca2+-activated factor (CAF) were isolated from bovine L, ST and PM muscles. Total CAF activity was high and similar in L and ST muscles, but PM muscle contained less than half the total CAF activity of L and ST muscles. Incubation of purified CAF with myofibrils isolated from at-death muscle caused Z-disk degradation and disappearance of troponin T and the simulataneous appearance of a 30,000 dalton component. That incubation of purified CAF with purified troponin caused degradation of troponin-T to a 30,000-dalton component indicates that the 30,000-dalton component in whole myotibrils originates from troponin-T. The effects of CAF on Z-disk and troponin-T degradation and the relative total activity of CAF in L, ST and PM muscles are similar to the effects of postmortem storage in myofibril fragmentation, myotibrillar protein degradation and WB shear force values. These parallel effects indicate that the limited and. specific proteolysis of myofibrillar proteins is caused by a Ca2+-activated factor endogenous to the muscle cell.