The purpose of this study was to investigate the properties of myofibril and myosin B. Samples were extracted from the longissimus dorsi of lumber vertebra from the porcine weighing over 110kg in live weight. The porcine muscle was held either at 0℃ or at 40 ℃ for 30, 60, and 90 min. The carcass temperature immediatly after slaughter ranged from 42.0℃ to 42.6℃ with the line weight. The samples held at 40℃ showed much less contractility than those treated at 0℃. No contractility reaction was found on those treated for 90 min at 40℃. Extractibility of myosin B from the sample treated at 40℃ was lower than those treated at 0℃ The myofibril was not found in the residue of the sample treated at 0℃ which myosin B was extracted from treated at O℃ But there was much more myofibril in the residue from those treated for 60 min and 90 min at 40℃. Thus, the difference found in this experiment seemed at to reflect the differences in biological activities of the myofibrillar protein. As they corresponded to PSE porcine muscle symptoms, it can be concluded that the porcine carcass has to be held at 0℃immediately after slaughter in order to produce good quality pork especially from the animals exceeding 110kg in live weight.