We aimed to investigate the effect of virus-spiking conditions on the filter performance (flux, flux decay, and parvovirus reduction) of the small virus filter Planova (TM) 20N. We used three kinds of porcine parvovirus (PPV) stocks: serum, serum-free, and purified. The flux profile with PPV spiking was similar to that without spiking for normal load filtration of about 250-300 L/m(2). High volume (3 vol %) of serum-free PPV and 1 vol % serum PPV reduced the flux to some extent for high-load filtration (over 10 h, ca., 500 L/m(2), 5 mg/mL IgG solution). Log reduction value (LRV) of PPV was maintained at a high level (>5) over the filtration volume. Flux for Planova (TM) 20N was only minimally affected by the use of different virus stocks for spiking. Transmission electron microphotography showed that the distribution of PPV particles captured inside the membrane wall was reached until the -60% thickness of the membrane, showing that the membrane of Planova (TM) 20N has a thick effective layer for virus removal. These results provided evidence for the robustness of the filter performance of Planova (TM) 20N, showing that it was not easily affected by virus spiking conditions and that it has a large capacity for high-load conditions. (C) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 27: 162-169, 2011