Nanofiltration is mainly used as a final step in the production of biologicals purified from human plasma and of recombinant DNA-derived products. Fetal calf serum is often filtered three times using a cut-off of 100 nm (removal of, for instance, IBR and PI-3, but not of BVDV). Only some companies provide fetal bovine serum which is serially filtered through 40 nm pore size filters (Hanson and Foster, 1997) , because this cut-off allows also the elimination of, for instance, BVDV, which has a size of 45-55 nm (see Table XIII ).