Fig. 1 shows the changes in activities of the calpain system extracted during frozen storage of meat at −20 and −80 °C in experiment 1. The stability was similar at the two storage temperatures for all three components of the calpain system. The extractable activity of μ-calpain and m-calpain did not change during the storage period indicating that the enzymes were stable at both storage temperatures. The activity of calpastatin was constant in the first few weeks of the storage period, thereafter the activity decreased until day 123. The decreased activity of calpastatin is in agreement with Koohmaraie, 1990, Whipple and Koohmaraie, 1992 and Duckett et al., 1998 and Ingolfsson and Dransfield (1991). The previous studies, however, observed a more rapid decrease in calpastatin activity during frozen storage than reported in Fig. 1. Meat samples are stored at −80 °C because chemical and biological events are expected to be negligible at this temperature. Thus, it is surprising that calpastatin seems to be unstable at this temperature and that no difference is observed between storage at −20 and −80 °C. The rates of chemical reactions are normally highly dependent on temperature. Fig. 2 shows that the activity of the m-calpain stock solution decreased during storage in 30% glycerol at 4 °C due to inactivation of the m-calpain molecules. The observed decrease in calpastatin activity (Fig. 1) was related to the change in activity of the m-calpain stock solution, i.e., the lower the activity of m-calpain stock solution, the lower the measured activity of calpastatin (Fig. 3). To test whether this relation was causal, the activity of calpastatin in meat stored at −80 °C for 143 days was assayed using both the original stock solution of m-calpain and a newly purified stock solution. The resulting calpastatin activity using the new stock solution was similar with the initial levels observed at the start of the storage period (Table 1). The new stock solution gave a 44% higher calpastatin activity at a meat storage time of 143 days than the original stock solution and there was no difference between the calpastatin activity originally measured at day 0 and calpastatin activity measured at day 143 using the new stock solution. Thus, the observed decrease in calpastatin activity during frozen storage of meat (Fig. 1) appears related to the instability of the m-calpain stock solution.