Changes in the activity of peroxidase (EC 1.11.1.7) were compared under the influence of two simultaneously present metal ions, one of which deactivates most enzymes (mercury or cadmium) while the other possesses stimulating properties (magnesium, manganese or calcium). A possibility of a considerable increase in peroxidase activity (50-200%) was found in the presence of the ions of manganese, magnesium or calcium, in spite of the presence of ions known to possess toxic properties (mercury or cadmium). An enzyme immobilization method was employed in order to determine the kind of protein functional groups (COOH or NH2) involved in the processes of peroxidase activity changes due to the presence of metal ions. It was found that by immobilizing peroxidase by free amino groups of the enzyme, one achieves a considerable decrease in the toxic effect of mercury or cadmium, and a higher increase in peroxidase activity in the presence of manganese, magnesium or calcium.