A method for purifying a desired heterologous polypeptide from microbial fermentation broth or homogenate in which it is produced and solubilized is described. This method involves adding to the broth or homogenate an effective amount of a solution of 6,9-diamino-2-ethoxyacridine lactate (ethacridine lactate) to precipitate host cell impurities under conditions wherein the majority of polypeptide remains soluble, and separating the desired polypeptide from the broth or homogenate. The broth or homogenate containing the ethacridine lactate and polypeptide is also disclosed.