It should be mentioned here, that the case of MVM contamination of CHO cultures for the production of TPA, did not lead to a cytopathic effect and could not be detected without specific virus tests (Garnick, 1996) . In contrast, Nettleton and Rweyemamu (1980) and Hughes (1996) reported on a MVM contamination of BHK-21 cells for veterinary vaccine production, which was detected via persistent cell deaths of these cells. It could be shown that the serum batch was the origin of this contamination. It is evident that rigourous testing of raw materials is necessary because of the following: -In the case of the production of recombinant proteins for human use, virus contaminations can only be eliminated with difficulty from the bulk product. Should a virus present in the bulk product be able to be eliminated during downstream processing, the FDA will generally not accept the final product after purification (Burstyn, 1996) . -The problem associated with live attenuated virus vaccines is that these products cannot be treated for virus inactivation because the active ingredient would be inactivated at the same time. Such products require a more extended quality control testing for the raw materials. For instance, new serum batches should be tested for a more extensive range of bovine viruses and in particular for those viruses, which are of relevance for the final application of the product. For instance, in the case of the contamination of a canine vaccine by bluetongue virus which lead to the death of some bitches (Table X) (Wilbur et al., 1994) , the application of a specific test would have avoided this incidence. -In conclusion, the best solution for reducing the risks of viral contaminations is the use of raw materials which are not of animal or human origin, but of plant or microbial origin or produced by chemical synthesis. It is evident that such an approach will not eliminate the risk of viral contaminations, but represents an important step towards risk minimization.