The effect of cadmium ions on calcium-dependent proteinases (calpains) in some organs of the blue mussel, Mytilus edulis L., was investigated in in vivo and in vitro experiments. It was shown that the exposure with cadmium ions affected calpain activity in time-dependent manner in an aquarium experiment. Increased calcium-dependent proteolytic activity in mussel gills was shown after 24 h exposure with cadmium. The observed calpain upregulation in mussel gills presumably is a constituent of non-specific compensatory response to acute heavy metal exposure, which allows the organism to adapt to the pollutants. Cadmium-induced inhibition of calсium-dependent proteolytic activity was observed after 72 h treatment. The effect of heavy metal ions on calpains in the blue mussel, Mytilus edulis L., was tested in vitro using partially purified mussel enzymes. Calpain activity was fully inhibited by cadmium ions. The mechanism of interaction of cadmium ions with the studied cysteine proteinases can be realized by specific binding with SH groups of the enzyme.