The in vivo and in vitro effects of weak, low-frequency magnetic fields with resonance parameters for calcium ions upon intracellular calcium-dependent proteinases (calpains) in the crucian carp (Carassius carassius (L.) and roach (Rutilus rutilus (L.) were studied. It has been revealed that the impact of a weak low-frequency magnetic field leads to considerable decrease in the activity of calpains in the fish skeletal muscles and brain.