Although previous studies have demonstrated the potential of using membrane-based processes for purification of plasmid DNA, there is considerable discrepancy regarding both the physical mechanisms governing DNA transmission and the effects of membrane pore size and operating conditions on the DNA sieving coefficient. The objective of this work was to obtain quantitative data on the transmission of a 3.0 kbp supercoiled plasmid DNA through Ultracel ultratiltration membranes with different nominal molecular weight cut-offs over a range of filtrate flux. The extent of plasmid transmission was a very strong function of the filtrate flux, with the sieving coefficient increasing from essentially zero to nearly one as the flux increased. Data were analyzed in terms of available filtration models to examine the contributions of DNA elongation, shear deformation, and concentration polarization on the observed plasmid transmission. The results clearly indicate the importance of flow-induced elongation or deformation on DNA transmission, with the data consistent with a modified version of the elongational flow model using the plasmid superhelix radius as the characteristic dimension. The results provide important insights into the factors controlling DNA transmission through semipermeable ultrafiltration membranes. (C) 2007 Elsevier B.V. All rights reserved.