A review. Virus filtration is routinely employed as part of a protein drug manufacturer's comprehensive virus safety strategy. Virus filters have been verified to be capable of providing consistent, high-virus clearance for viruses larger than the claimed filter rating, with variable clearance for viruses below the rating. In line with this, the filter manufacturer and end users developed filter tests methods for lot characterization and virus removal validation studies, and in 2002 the Parenteral Drug Association (PDA) organized the PDA Virus Filter Task Force to formulate a tech. report supported by scientific data for end users having standardized test methods for classifying and identifying viral-retentive filters, grounded by a common nomenclature. The classification of viral retentive filters is also described. Generally, the PDA Virus Filter Task Force has been regarded as a set new optimized test methods for both small and large virus filters.