by Barbara Tipton, Jeri Ann Boose, Joanne Beck, Thomas O'Brien, and William Larsen, CUNO Incorporated Two types of depth filters are studied for their ability to rid protein solutions of retroviruses and parvoviruses. Such filters are relatively economical and easy to use, so as secondary virus-removing devices, they are beneficial for downstream processing of mammalian cell cultures. Results indicate that depth filters are more effective on retroviruses than parvoviruses.