Herein is reported a method for purifying cell cultivation supernatants either directly after fermentation or after one or more preliminary purification steps, such as protein A affinity chromatography. By adjusting the pH value in the acid range and subsequent incubation of the acidified solution host cell nucleic acid and host cell protein can be precipitated but the target polypeptide remains in solution. Thereafter the precipitate and therewith the contaminating host cell components can be removed by a simple physical separation step.