We tested the ability of hollow-fiber ultrafilters with molecular weight cut-offs (MWCOs) of 50 000, 13 000, and 6000 to remove and detect viral agents (phage T1, 50–150 nm; phage PP7, poliovirus, 28–30 nm) from ultrapure water, 0.85% saline with 1% trypticase soy broth, and Dulbecco's modified Eagle minimum essential medium with 10% fetal bovine serum (DMEM-10). Virus diluted in saline and DMEM-10 were tested to evaluate filter performance under conditions that minimize the adsorption of viral particles to the filter matrix. During filtration, the retentate was returned to the input reservoir, and the permeate was removed to a separate vessel. Thus, the virus concentration in the feed increased over the course of filtration. Filter performance was evaluated by comparing the concentration of infectious virus in the initial virus suspension with the virus concentration in the permeate and retentate. Very efficient removal of phages T1 and PP7 was observed with the filters with MWCOs of 13 000 and 6000 (titer reduction > 7 logs) for all three fluids tested. No poliovirus was detected in the permeate of the ultrafilters with MWCOs of 13 000 or 6000 (titer reduction > 6 logs). These results indicate that the ultrafilters with MWCOs of 13 000 and 6000 were very effective in removing small viral particles (25–30 nm) by size exclusion. The recovery efficiency of the virus in the retentate varied by fluid type. However, filtration with virus diluted in DMEM-10 resulted in consistent recovery of the viruses tested. The results suggest that these ultrafilters may have the dual potential of removing viral contaminants from fluids and concentrating virus in the retentate.Key words: virus removal, virus concentration, ultrafiltration membranes.