Therapeutic proteins such as monoclonal antibodies are produced in cell culture, where impurities can result from the host cells or substrates. Examples of these impurities are host cell protein and host cell DNA. In any purification process, it is desirable to remove these impurities as early in the process as possible. This 3M Purification Inc. application brief addresses removal of DNA using cellulosic depth filters. Depth filtration is often employed for the first purification step, separation of cell mass from therapeutic proteins.