SUMMARY:The effects of muscle tenderness classification and of aging muscle postmortem on ATPase activity and superprecipitation of natural actomyosin were studied. Actomyosin from muscle 12‐ and 24‐hr postmortem had higher ATPase activity than that from 0‐hr, 5‐day aged or 10‐day aged muscle. However, ATPase activity did not usually return to the 0‐hr level. No consistent differences were found in actomyosin ATPase activity after the various periods of aging for actomyosins from tough and tender muscle. Superprecipitation of actomyosin was used as a measure of contractility. Actmyosin from 12‐ and 24‐hr postmortem muscle superprecipitated faster than that from 0‐hr muscle. However, actomyosin from 5 and 10‐day aged muscle superprecipitated less rapidly than that from 12‐ and 24‐hr postmortem muscle. Superprecipitation was more rapid in actomyosin from tough muscle than tender muscle at low KCI concentrations, but this was not true at high KCI concentrations. This observation suggested that actomyosin from tough muscle had a stronger interaction or higher amounts of some protein factor such as α‐acti
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