In general, the unprocessed bulk material (pool of harvests of cells and culture media) should be tested for viruses after the end of production and before any downstream processing. The scope, extent, and frequency of virus testing on the unprocessed bulk should be determined by taking several points into consideration including the nature of the cell lines used to produce the desired product, the results and extent of virus tests performed during the qualification of the cell line, the cultivation method, raw material sources and results of viral clearance studies. In vitro screening tests, using several cell lines, are generally employed for testing. If appropriate, a PCR test or other suitable methods may be used. The presence of an adventitious virus will block the further use and processing of the harvest material.