ABSTRACTMyofibrils were isolated in a buffered medium by differential centrifugation from at‐death and 1‐, 2‐ and 10day postmortem bovine longissimus muscle. Following isolation, myofibrils were extracted with Hasselbach‐Schneider (H‐S) solution, 0.6M KC1 buffered solution and 1 mM Tris, pH 8,5 solution. Postmortem changes in isolated myofibrils and their protein extracts were characterized with SDS‐polyacrylamide gel electrophoresis. SDS‐polyacrylamide gels showed that H‐S and KC1 buffered solutions extracted thick and thin filament proteins of myofibrils isolated from either at‐death or postmortem muscle samples. Changes in protein extractability with H‐S and KC1 solutions during postmortem storage were difficult to assess. 1 mM Tris solution extracted the thin filament proteins, actin, tropomyosin and troponin, and the thick filament proteins, C‐protein and myosin light chains. α‐Actinin was very resistant to 1 mM Tris, pH 8.5 solution extraction from isolated myofibrils of at‐death muscle, but it was readily extract‐able from isolated myofibrils of postmortem muscle. C‐protein also seems to become more extractable during postmortem aging of muscle. The prominent and distinguishable change occurring in the strong salt‐soluble proteins of the myofibril during postmortem storage of muscle at 2°C was the gradual degradation of troponin T and the concurrent appearance of a 30,000‐dalton component. These results indicated that modifications occur in thick and thin filament proteins during postmortem storage and that these modifications are possibly related to calcium activated factor (CAF).