The detection of adventitious viruses in cell banks has to follow two principles -use of detection methods for specific viruses such as MAP, HAP, RAP (mouse, hamster, rat antibody production tests -examination of serum antibody levels against specific viruses or enzyme activity after a specified period), and different specific PCRs, as well as the use of general tests which may indicate the presence of one or more of a large variety of different viruses. General tests include the in vitro test for adventitous virus. This test involves the inoculation of different cell lines -a human, a primate and a bovine (if bovine material was used for the production, otherwise a cell line from the species of origin of the cell substrate), and the production cell line (co-cultivation test). The RTase assay is a general test which will detect the presence of all viruses which contain reverse transcriptase enzyme. The last general test which is normally applied is the, in vivo tests where animals are used to identify the presence of virus. The animals are treated using different inoculation routes (the health of these animals should be monitored and any abnormality should be investigated to establish the cause of the illness). Finally, electron microscopic examination is also a general test which can be used for detecting adventitious viruses in the case of rather high virus loads. More details can be found in the articles by Poiley (1990) , by Werz et al. (1997) , and in the ICH Topic Q5A (1997).