The aim of the presented work was to investigate the stability of biocompatible magnetic uid, i.e. water-based magnetic uid containing magnetite nanoparticles stabilized by surfactant sodium oleate and modied by bovine serum albumin (BSA) after electron irradiation. Samples with the same concentration of Fe3O4 but dierent mass ratio BSA/Fe3O4 (w/w= 0.25, 1.0 and 2.5) were studied. The electron irradiation caused about 10% reduction of the saturation magnetization in the samples with w/w BSA/Fe3O4 ratio of 0.25 and less than 5% in the samples with w/w BSA/Fe3O4 ratio of 1 and 2.5.