ABSTRACTThis study was carried out to determine the effects of postmortem storage time, temperature and pH on myofibrillar proteins of red and white muscle. Myofibrils were isolated from (1) the red and white portion of semitendinosus muscle postmortem stored at 2°C and (2) at‐death red and white portions and suspended and stored in unbuffered 0.15M KC1 at 2°C and in buffered 0.15M KC1 (pH 5.5 and 7.0) at 2° and 25°C. To determine the effect of storage conditions on myofibrillar proteins, ATPase activity was assayed at different ionic strengths and with different modifiers. Assays of myofibril ATPase activity from postmortem muscle showed that (1) myofibrils from the white portion had greater ATPase activity than those from the red portion, (2) Ca2+‐modified activity from both portions increased and (3) Mg2+‐EGTA‐modified activity increased from the white portion, but remained unchanged from the red portion, during postmortem storage. These changes could be due to modifications of the regulatory protein components of muscle by calcium‐activated factor activity. For those myofibrils isolated from at‐death muscle and incubated under simulated storage conditions, a precipitous decrease occurred in Ca2+‐and Mg2+‐(low ionic strength) and Ca2+‐ and EDTA‐(high ionic strength) modified ATPase activity of myofibrils stored in 0.15M KC1, pH 5.5, at 25°C. Otherwise, little change occurred in these activities under other simulated conditions of storage (i.e., 2°, pH 5.5 and 7.0; and 25°, pH 7.0) with the exception that EGTA modified activity (indicates loss of Ca2+sensitivity) increased from the white portion at 25° and 2°C, pH 7.0, and from the red portion at 25°C, pH 7.0. Hence, a high storage temperature of 25°C has more detrimental effect on the integrity of myofibrillar proteins, as measured by changes in ATPase activity, than does