It is evident that a research laboratory cannot afford all tests, which have to be performed by the biotech industry in the case of GMP production. In addition, there is no need for such exhaustive testing unless the materials are to be used in the treatment of human patients. However, most of the issues described in the chapters on 'Problems associated with viral contaminations' and on 'Origin of viral contaminations' are valid for everyone working in the field of animal cell culture. The use of validated cell lines, shown to be 'virus-free', is the best choice because cell collections, such as the ATCC (www.atcc.org), perform entrance tests for new cell lines for assuring the absence of mycoplasma, bacteria, fungi, protozoa, and cytopathic viruses and can guarantee a certain microbial quality for the delivered cell lines. The German Cell Line Bank (www.dsmz.de) provides cells, most of them have been tested for the absence of HIV-I, HTLV-I and II, EBV, HBV, HCV, and HHV-8 by using PCR or RT-PCR. The absence of retroviruses is proven by performing a reverse transcriptase assay.