Membrane Ultrafiltration Membrane ultrafiltration has long been recognized as a powerful and reliable technique for bioseparation, concentration, or reformulation of proteinaceous so­ lutions. Many examples can be found in the literature for concentrating viruses from very dilute solutions using dead-end or tangential flow filtration [1-3]. At some point along the way, scientists realized that ultrafiltration could not only be used to concentrate viruses but also that it could remove them from the final product by retaining virus while allowing product to pass through the filter. Re­ cent uses of ultrafiltration for removing virus particles from solutions are cited below. This is not intended to be a thorough review of the literature, but rather a representative guide of virus removal by filtration.