Steady state kinetics of DNA depolymerisation in the presence of the DNAase A and Mg2+ ions were investigated at pH 5.5 and wide region of the enzyme, substrate and metal ion concentrations. A model, which is consistent with experimental results obtained is suggested. According to the model catalytically active form of the DNAase A should be a metal-bound enzyme. That species reacts with the metal-free DNA to form the Michaelis complex. The kinetics observed can be described in terms of mechanism which involves covalent enzyme-substrate intermediate formation. It was shown that the second Mg2+ ion binding to the complex Mg2+ DNAase -- DNA (KD - 2.2 . 10(-3) M) enhances the kinetic parameters of the reaction. To rationalise the effect one has to assume that the rate of the intermediate formation was accelerated as a result of the second Mg2+ binding.