-In contrast to bacterial and fungal contamination, viral contamination cannnot be easily detected, because they cannot be observed by normal light microscopy. It is only when a viral contamination leads to a morphological modification of the cultured cells, such as a cytopathic effect, that a contamination by a virus can be suspected. Silent infection by viruses with no observable morphological modification of the infected cell are clearly of greater concern. -A restricted number of viruses can infect cells and can integrate as a provirus, as in the case of Adeno-Associated virus (AAV), for instance. In this case, the provirus is present, but cannot replicate without the assistance of a helper virus, when one is present both virus species will be produced, together (Mayor and Melnick, 1966; Mayor et al., 1967; Hoggan et al., 1972; Berns et al., 1975) . -Virus contaminated cell cultures represent a risk for the operators, collaborators, patients, as well as for non-contaminated cell cultures. -Cells contaminated with some viruses can show a change to their susceptibility to infection by other viruses. For example some safety testing proto-cols require indicator cells to be used to show the presence of virus, if these are chronically infected by viruses this reduces their susceptibility to other virus species, this in turn, can lead to false negative results, because the virus to be detected can no longer infect the indicator cells. -As a general statement, cell lines contaminated by viruses cannot be treated to become virus free. The result of this is that potentially valuable cell lines will have to be discarded and replaced by new, non-contaminated cells. One of the few exceptions to this rule is the case of Lactate Dehydrogenase virus (LDV). Cultures of cells recovered from a passage in infected animals contain this virus; however, as this virus cannot infect the cells, it will be diluted during subsequent in vitro passaging and thus will be lost (Nicklas et al., 1993; Nakai et al., 2000) .