Improved virus retention can be obtained by using pore cut offs of 35 nm. Using a 35 nm membrane in line with two prefilters (one 75 nm filter followed by a first 35 nm filter) led to log titer reductions of >4.3 for Hepatitis A Virus (HAV) and Encephalomyocarditis Virus, although both viruses are smaller (28-30 nm) than the cut-off of the filters (Troccoli et al., 1998) . All viruses larger than 35 nm were completely removed.