In this study, magnetic nanoparticles Fe3O4synthesized by the polyol method were used to immobilize Horseradish root peroxidase. The surface of magnetic nanoparticles was pretreated with tetra-ethyl orthosilicate, 3-aminopropyltriethoxysilane, and glutaraldehyde. As a result, two biocatalysts were obtained Fe3O4/APTS/GA/HRP and Fe3O4/TEOS/APTS/GA/HRP. The activity of the synthesized biocatalysts was estimated spectrophotometrically in the oxidation reaction of 2,2'-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) with hydrogen peroxide. The optimal conditions for this reaction were selected (pH 6.0, temperature 45 °C). It has been determined that biocatalysts Fe3O4/APTS/GA/HRP and Fe3O4/TEOS/APTS/GA/HRP slightly decreased their activity during 5 consecutive experiments, by 19 and 15 %.