A vibratory shear-enhanced filtration system (VSEP) was used for ultrafiltration of UHT skim milk and a skim milk reconstituted from low-heat powder, which has a similar protein content as fresh milk. Two polyethersulfone membranes of 10 and 50 kDa cutoff were used, respectively, for total protein concentration and for alpha-lactalbumin (alpha-LA) separation from beta-lactoglobulin (beta-LG). With the 50 kDa membrane, casein micelles were completely rejected after 40 min of filtration while alpha-LA transmission rate (ratio of permeate to retentate concentrations) remained between 22% and 28%. The beta-LG transmission rate was around 1% and the stabilized permeate flux at initial concentration (Volume Reduction Ratio, VRR = 1) and a transmembrane pressure of 250 kPa was between 60 and 70 Lh(-1) m(-2). Permeate flux data on this membrane were found to be very close to those obtained under the same conditions with UHT skim milk until a VRR of 3. At higher concentrations, the reconstituted milk yielded higher permeate fluxes, 29 L h(-1) m(-2) at a VRR of 6 vs. 15 L h(-1) M for UHT milk. The 10 kDa membrane retained whey proteins completely and yielded higher fluxes-up to 82 L h(-1) m(-2) at initial concentration and a transmembrane pressure (TMP) of 1500 kPa-than data reported for conventional ultrafiltration. The maximum VRR reached in concentration tests was 13 with a final flux of 8 L h(-1) m(-2), corresponding to a maximum theoretical VRR by extrapolation to zero flux of 17. The critical shear stress at which a steady TMP could be maintained for a constant flux of 30 L h(-1) m(-2) was found to be 13.7 Pa.