ABSTRACT: This study was conducted to test the hypothesis that postmortem calcium injection could activate the calpain system in mature Chinese Yellow Cattle muscle, thereby promoting meat tenderization through disruption of the myofibril structure during aging. A 10% (w/w) injection of CaCl2 (300 mM) solution lowered the Warner-Bratzler shear values of longissimus muscle by more than 30% (P < 0.05), even with only 24 h postmortem storage when compared with noninjected or water-injected controls. The accelerated meat tenderization by the Ca2+ treatment paralleled the changes in myofibril fragmentation index and fracture of the myofibril ultrastructure throughout the sarcomere but most notably around the I-bands and the Z-disks. Injection of ZnCl2 (50 mM) largely inhibited these proteolytic changes. The colorimetric L* and a* values were not affected by CaCl2 nor by ZnCl2 injection. The results suggest that postmortem CaCl2 injection can be used to help resolve the toughness problem of mature Chinese Yellow Cattle meat and shorten the aging time required to achieve adequate tenderness.