Ultrafiltration can potentially be used for the concentration and purification of plasmid DNA. The objective of this study was to determine the role of electrostatic interactions on the transmission of a supercoiled plasmid through negatively charged composite regenerated cellulose membranes produced by covalent attachment of sulphonic acid groups to the base cellulose. The plasmid sieving coefficient at any given flux decreased with increasing membrane charge. Plasmid transmission increased with increasing filtrate flux due to flow-induced plasmid elongation, with the critical flux for plasmid transmission increasing for the more negatively charged membranes. The intermolecular electrostatic interactions could be shielded by using high ionic strength solutions, with very similar plasmid transmission obtained with the unmodified and negatively charged membrane under these conditions. The experimental results were analyzed using a partitioning model that accounts for the flow-induced elongation of the plasmid DNA in the converging flow field above the membrane pore, providing new insights into the physical phenomena governing plasmid transmission in ultrafiltration. (C) 2009 Elsevier B.V. All rights reserved.