A method for purifying a protein from a composition containing an impurity that comprises the steps: (a) Adding polyethylene glycol to the composition； (b) Adding zinc to the composition； (c) Allowing the protein to form a precipitate following any order of steps (a) and (b)； (d) Collecting the precipitated protein. The polyethylene glycol and zinc are the only precipitating agents used in the method. The method can be used to purify a recombinant antibody.
