The European Union (EU) guideline concerning the virus safety evaluation of biotechnological investigational medicinal products (CPMP/BWP/398498/2005) recently came into force. In the guideline it is stated that analytical procedures supporting the qualification of cell banking systems, starting materials, as well as testing of unprocessed bulks for the presence of adventitious viruses, should be supported by appropriate qualification/validation studies. The validation protocols should prospectively set claims for assay performance, which should be verified by the validation experiments and demonstrate that a particular procedure is suitable for its intended purpose. Assay parameters for adventitious virus testing are discussed, and examples of validation of qualitative and quantitative assays for the detection of blood-borne viruses in human plasma are considered.