This paper investigates whether UHT skim milk could be a valid fluid model for testing the separation of whey proteins from casein by microfiltration (MF) and of alpha-Lactalbumin (alpha-La) from beta-Lactoglobulin (beta-Lg) by ultrafiltration (UF). To this effect, we have measured permeate fluxes, alpha-La and beta-Lg transmissions during MF concentration tests with pasteurized and UHT skim milks using a dynamic filtration module with rotating ceramic membranes. Then, wheys resulting from these permeates were ultrafiltered at 50 kDa with a rotating disk module while measuring permeate fluxes and alpha-La and beta-Lg transmissions during concentration. Results showed that, in MF, permeate fluxes were slightly higher for UHT milk than for pasteurized milk. Student tests showed that, if alpha-La transmissions were slightly higher for UHT milk than for pasteurized milk, they were not significantly different. beta-Lg transmissions were a little higher for UHT than for pasteurized milk. In UF tests, permeate fluxes were higher at low VRR (volume reduction ratio) for pasteurized whey, but decayed faster than for UHT milk when VRR increased. alpha-La and beta-Lg transmissions in UHT whey were not significantly different from corresponding ones in pasteurized whey. These results show that the lower concentration of native whey proteins in UHT milk does not affect much their transmission and permeate flux, both in MF and in UF. (C) 2010 Elsevier B.V. All rights reserved.