However, the best means to increase the biological safety of the produced viral vaccines is the use of diploid or continuous cell lines, because it can be determined that such cells are free of animal derived viruses: This can be achieved by establishing master (seed stock) and working (distribution and user stocks) cell banks which have been rigorously tested and validated for the absence of microbial as well as viral contaminants (see chapter by Freshney and the section on 'Testing-virus screening in cell banks' of this article). By this means producers of viral vaccines and all other biotech products can make use of a homogeneous pool of characterized cells from which each production run will be started, in the knowledge that they are free of any contaminant (because they have been tested) (Berthold et al., 1996) . In addition, by using the seed stock/working stock concept for the viral inoculum the manufacturer of viral vaccines can use a tested and validated stock of virus inocula of which one aliquot is used for the infection of each production run.