Disclosed is a method for purifying at least one target substance that is to be identified and is present or is formed in a cell culture medium when cells are cultivated. In said method, magnetic particles, i.e. beads, to the functionalized surface of which the target substance selectively attaches, are added to the cell culture medium, and the particles to which the target substance is attached are selected out of the cell culture medium by applying a magnetic field. The method is characterized by the following steps: a serum substitute is provided that is obtained from a natural serum and is free or virtually free of low-molecular substances having a maximum mass of 60 kDa, particularly a maximum mass of 10 kDa; the serum substitute is admixed to the cell culture medium which already contains the cells or to which the cells are added; the cells are incubated in the cell culture medium enriched with serum substitute; at least some of the cell culture supernatant formed during the incubation is separated; the cell culture supernatant is filtered by means of an ultrafiltering process so as to obtain a retentate; the beads are supplied in such a way that the functionalized surface of the beads comprises a plurality of dendrimers containing up to 10 branches, i.e. ten generations, the terminal points of the last generation of each dendrimer being modified; the beads and the retentate are admixed to a buffer solution so as to obtain a mixture; the target substances contained in the retentate are incubated and are fixed to the beads; and the magnetic beads are magnetically selected out of the mixture.