Although it would be desirable that all raw materials should be tested for the absence of adventitious agents, in order to be sure that they are safe, this is often impracticable. Therefore there are two approaches: first, tests are employed which are based on the detection of general characteristics of viruses (cytopathic effects, haemadsorption) and, second, specific tests using imunological and/or PCR methods are employed for detecting virus antigens or specific viral sequences after amplification in permissive cells (see Table IX for testing of bovine serum and porcine trypsin, see Section on 'Testing -virus screening in cell banks').