Ultra scale-down (USD) approach is a powerful tool to predict large-scale process performance by using very small amounts of material. In this article, we present a method to mimic flux and transmission performance in a labscale crossflow operation by an USD rotating disc filter (RDF). The Pellicon 2 labscale system used for evaluation of the mimic can readily be related to small pilot and industrial scale. Adopted from the pulsed sample injection technique by Ghosh and Cui (J Membr Sci. 2000;175:5-84), the RDF has been modified by building in inserts to allow the flexibility of the chamber volume, so that only 1.5 mL of processing material is required for each diafiltration experiment. The reported method enjoys the simplicity of dead-end mode operation with accurate control of operation conditions that can mimic well the crossflow operation in large scale. Wall shear rate correlations have been established for both the labscale cassette and the USD device, and a mimic has been developed by operating both scales under conditions with equivalent averaged shear rates. The studies using E. coli lysate show that the flux vs. transmembrane pressure profile follows a first-order model, and the transmission of antibody fragment (Fab') is independent of transmembrane pressure. Predicted flux and transmission data agreed well with the experimental results of a labscale diafiltration where the cassette resistance was considered. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 26: 466-476, 2010