The invention concerns a method for isolating proteins and/or protein and nucleic acid associations in a sample, comprising steps which consist in: contacting said sample and magnetic colloidal particles comprising a core and a coat wherein: the core is magnetic and is coated with at least a polymer comprising functional groups X selected among amine, hydroxyl, thiol, aldehyde, ester, anhydride, acid chloride, carbonate, carbamate, isocyanate and isothiocyanate groups or mixtures thereof, whereof at least one fraction has reacted with other functional groups of the coat, and the coat consists of a polymer bearing functional groups Z and Z', capable of ionisation, identical or different, selected among amine, carboxylic acid, ester, anhydride, aldehyde, thiol, disulphide, .alpha.-halogenocarbonyl, sulphonic acid, maleimide, isocyanate and isothiocyanate groups to form a mixture; incubating said mixture in predetermined conditions; and separating from the mixture the protein and/or protein and nucleic acid associations complexed on the colloidal particles by applying a magnetic field. The invention also concerns a complex consisting of colloidal particles and proteins, a reagent comprising such a complex or colloidal particles.