The magnetic colloidal particles comprise a core and an envelope in which the core is magnetic and is coated with at least one polymer comprising functional groups X chosen from amine, hydroxyl, thiol, aldehyde, ester, anhydride, acid chloride, carbonate, carbamate, isocyanate and isothiocyanate groups, or mixtures thereof, at least one fraction of which has reacted with other functional groups of the envelope, and the envelope comprises a polymer bearing ionizable functional groups, Z and Z', which may be identical or different, chosen from amine, carboxylic acid, ester, anhydride, aldehyde, thiol, disulfide, .alpha.-halocarbonyl, sulfonic acid, maleimide, isocyanate and isothiocyanate groups, which have partially reacted with the functional groups X of the core. These magnetic colloidal particles can be used to isolate biological material.