Mammalian cell-expressed therapeutic proteins are particularly vulnerable to contamination by endogenous retrovirus-like particles (RVLPs). The Viresolve NFR filter was designed to meet the critical requirement of manufacturing a safe and virus-free therapeutic by retaining RVLPs by a minimum of six log reduction value (LRV). The NFR designation refers to retrovirus removal in a normal flow format. To qualify the product, we tested two model viruses: the 78 nm diameter 06 bacteriophage and the 80-110 nm diameter Xenotropic Murine Leukemia Virus (X-MuLV). Robust retention was demonstrated over a wide range of process parameters. Viresolve NFR filters also retain other model adventitious viruses including 70-85 nm diameter Reovirus 3 (Reo3), 70-90 nm diameter Adenovirus 2 (Ad2), and 53 nm diameter PR772 by >6 LRV. In addition to these model viruses, the filter retains >7 LRV of both the mycoplasma Acholeplasma laidlawii and the bacterium Brevundimonas diminuta. Protein passage is shown to be consistently high (95-100%) for a variety of therapeutic protein products, including monoclonal antibodies. Characterization of the filter in specific applications is made simple by availability of ultralow surface area (5 cm(2)) disks, which are shown to scale linearly to the manufacturing scale pleated-filters. Viresolve NFR filters provide consistent water permeability performance (34-37 LMH/ psi) and show very little plugging for all feedstocks evaluated. The Viresolve NFR filter incorporates Retropore, a unique asymmetric polyethersulfone membrane, the surface of which has been modified to minimize protein binding.