The characteristics of calpain II (EC 3.4.22.17) were investigated in order to clarify the function of this enzyme in post-mortem fish muscle. Calpain II purified from tilapia muscle (Tilapia nilotica × Tilapia aurea) was activated by Ca, Sr, Ba, and Mn, but inhibited by Fe, Co., Ni, Cu, Zn, Cd and Hg ions. This proteinase was unaffected by Na, K and Mg ions. The calcium ion might induce conformational changes and consequently activate this proteinase. The optimal pH and temperature for the activity were 7.5 and 30°C, respectively. Water activity and the ionic strength of the reaction solution significantly affected the activity of calapin II. Increasing ethylene glycol and polyethylene glycol significantly decreased the activity of calpain II. However, no significant effect on calpain II activity was apparent from the addition of nucleotides and their related compounds.