Magnetic bead cellulose particles and magnetic poly(HEMA-co-EDMA) microsheres with immobilized DNaseI were used for degradation of chromosomal and plasmid DNAs. Divalent cations (Mg2+, Ca2+, Mn2+ and Co2+) were used for the activation of DNaseI. A comparison of free and immobilized enzyme (magnetic bead particles) activities was carried out in dependence on pH and activating cation. The maximum of the activity of immobilized DNaseI was shifted to lower pH compared with free DNase I. DNase I immobilized on magnetic bead cellulose was used 20 times in the degradation of chromosomal DNA. Its residual activity was influenced by the nature of activating divalent cation. The immobilized enzyme with decreased activity was reactivated by Co2+ ions.