(e) Replacement of animal derived substances by non-animal/non-human derived substances: Although viral screening tests are efficient for detecting adventitious viruses, the best remedy for avoiding the presence of these viruses is the use of non-animal/nonhuman derived raw materials. This is not a complete assurance for the absence of virus, but a considerable risk reduction, since adventitious viruses might also be introduced via non animal derived raw materials, such as medium components as shown by Garnick (1996) . In this context it should be mentioned that most of the recent biologicals based on the use of animal cell technology are produced in serum-free or protein-free media (Froud, 1999; Merten, 1999) . With respect to the production of viral vaccines, the first serum-free viral vaccines were developed and tested in clinical studies (Brands et al., 1999; Kistner et al., 1999) , and are going to be put on the market.