Dextran sieving (or retention) tests have become a standard method for characterizing the pore size distribution of ultrafiltration membranes. The objective of this work was to extend this methodology to the analysis of parvovirus filtration membranes with much larger pore size than traditional ultrafiltration membranes. Experiments were performed with single layer membrane discs of the Pall Ultipor (R) DV20 and Millipore Viresolve (R) Pro filters using large polydisperse dextrans. The sieving coefficients for the Viresolve (R) Pro were dramatically different in the skin-side up and skin-side down orientations, consistent with the highly asymmetric pore structure of this membrane. The Ultipor (R) DV20 also shows some asymmetry in the sieving behavior, demonstrating that these membranes have a small gradation in pore size between the dull and shiny sides. Quantification of the sieving data was complicated by the large degree of peak spreading in the size exclusion chromatography columns and the polydispersity of the dextran standards. The experimental results and model calculations demonstrate the potential of using this modified dextran sieving test as a complement to virus and particle challenge tests to study the pore size characteristics, underlying pore morphology, and retention behavior of parvovirus filtration membranes. (C) 2011 Elsevier B.V. All rights reserved.