Purification of virus particles and viral vectors for viral vaccines and gene therapy applications is a major large-scale separations challenge. Purification of parvovirus particles such as adeno-associated virus, the leading candidate for gene therapy applications, is particularly challenging given their small size, typically 18-26 nm. We have investigated the use of ultrafiltration for purification of Aedes aegypti densonucleosisvirus, a mosquito parvovirus. Four ultrafiltration membranes with molecular weight cut offs of 30, 50, 100 and 300 kDa have been tested in tangential flow mode. Experiments have also been conducted where part of the permeate is pumped co-current to the feed. This results in a more constant transmembrane pressure drop along the entire membrane. Maintaining a more constant transmembrame pressure results in improved passage of host cell proteins and smaller virus particles. Currently chromatographic separations such as ion exchange are used to remove DNA and host cell proteins. Our results indicate that by carefully selecting the molecular weight cut off of the membrane and maintaining a constant transmembrame pressure along the membrane, virus particles may be purified and concentrated by ultrafiltration. Reducing the impurity load to subsequent chromatographic steps could result in the use of smaller columns leading to cost savings. (C) 2008 Elsevier B.V. All rights reserved.