Some treatments are presented in the following: (a) Nanofiltration (Troccoli et al., 1998; Aranha-Creado et al., 1997; Graf et al., 1999) . If the size difference (molecular weight) between the raw material and the virus is large enough, viruses can be removed by nanofiltration, which makes use of pore cut offs of 50, 35 nm, and even 15 nm. Filters with a pore cut offs of 50 nm can be used to eliminate viruses which have a diameter larger than 50 nm, such as retroviruses or influenza A virus (80-120 nm) (typical log titer reduction in a validation study: >6.3). However, such filters only partly reduce the quantity of poliovirus (28-30 nm), and viruses of a size of 25 nm (model particle: Bacteriophage PP7) pass without any significant retention (log titer reduction: <1-8.5, depending on the buffer system used) (Graf et al., 1999) . It should be mentioned that the composition of the medium/buffer system in which the virus is placed, has an effect on the log titer reduction of viruses which are below the molecular weight cut-off of the membrane used. An example of typical virus retention data for a commercial hydrophilic PVDF membrane filter is shown in Table XII .