The present invention provides method for purifying a recombinant protein from a gram-negative bacterial host cell sample or extract thereof wherein said host cell expresses a recombinant protein and a recombinant disulphide isomerase DsbC; comprising: a. adjusting the pH of the host cell sample or extract thereof to a pH of 5 or less to precipitate the recombinant disulphide isomerase; and b. separating precipitated recombinant disulphide isomerase DsbC from the recombinant protein to produce a recombinant protein sample.