(d) Other treatments: Substances of animal origin, such as serum or trypsin, or final biotech products can also be treated by other methods for reducing the eventual viral burden. These treatments can be of chemical nature, such as treatment with peracetic acid (Hughes, 1996) , solvents (e.g. 1% Tween 80 and 0.3% tri-n-butyl-phosphate at 25 • C for 8.5 h for the treatment of plasma derived Factor IX, works only for lipid enveloped viruses) (Johnston et al., 2000) , or imines (Brown et al., 1999) , or physical methods, such as heating (Hughes, 1996) or the reduction of the pH to 4.5. With respect to heat treatment, it is less effective than irradiation methods (Willkommen et al., 1999) and the serum composition is too much altered (Hanson and Foster, 1997) , leading to a rather weak growth promotion. The addition of a chemical substance, such as peracetic acid, is not ideal because a chemically reactive substance is added which might also lead to an inactivation of some medium compounds. In spite of this, virus inactivation based on the treatment with peracetic acid is rather effective for inactivating resistant virus, such as poliovirus Table XV . The effect of increasing doses of gamma radiation on the ability of fetal bovine serum to support the long term growth of selected cell lines in culture (expressed as percent of growth of control cultures) (Plavsic et al., 1999) (Sprössig and Mücke, 1967) , as well as for maintaining the growth supporting ability of the serum. Other adventitous agents like mycoplasmas and bacteria are also efficiently inactivated (Schweizer et al., 1972) .