The transmission of a 3.0 kbp supercoiled plasmid DNA through ultrafiltration membranes has recently been shown to be strongly dependent on the filtrate flux due to the elongation of the large DNA molecule in the converging flow field above the smaller membrane pore. The objective of this study was to extend these investigations to different size plasmids and to develop an improved theoretical model describing the effects of plasmid size, pore size, operating temperature, and solution viscosity on the critical filtrate flux for plasmid transmission. Experiments were performed in a stirred ultrafiltration cell using composite regenerated cellulose membranes. The critical filtrate flux, evaluated as the lowest filtrate flux at which plasmid transmission becomes significant, decreased with increasing membrane pore size but was essentially independent of the plasmid size (from 3.0 to 17 kbp). The critical filtrate flux was also a function of the operating temperature and glucose concentration through their effect on the viscosity of the buffer solution. The experimental results were in good agreement with predictions of a new model accounting for the effects of the DNA persistence length on the elongation of the supercoiled plasmid in the converging flow field into the membrane pore. (c) 2008 Elsevier B.V. All rights reserved.