-Complete testing can be impracticable on a batch to batch basis. In most cases, screening will only be done for certain viruses, e.g. for BVDV, IBR, and PI-3 in the case of bovine serum, for porcine parvovirus in the case of trypsin, because these are the most probable viral contaminants. However, depending on the geographical origin of the serum or the trypsin, additional tests for viruses which are present in that geographical area from where the raw material is coming may have to be performed (Table IX) . If a raw material is of animal origin, screening tests should also include the use of cells of the species of origin. With respect to the use of serum for the GMP production of biologicals for human use, the EMEA proposes in a draft that more viral screening tests should be performed for proving the absence of BVDV, IBR, PI-3, Bovine adenovirus, Bovine Parvovirus, Bovine Respiratory Syncytial virus, Bovine Re- virus tests, otherwise they pass undetected. -Due to sampling size, low titers of some adventitious viruses can remain undetected but may be amplified during the manufactureing process. In this context it should be remarked that screening methods are not always sufficient because contaminated serum batches which had passed as uncontaminated have been detected by and Yanagi et al., (1996) ; contamination of serum samples with BPyV and BVDV, respectively.