The tendency of calcium to promote microfiltration (MF) membrane fouling is well documented, but the role of lactose has not been studied. Milk protein concentrate that is 85% protein on a dry basis (MPC85) contains less calcium and lactose than skim milk (SM). Our objectives were to determine the limiting fluxes (LF) and serum protein (SP) removal factors (SPR) of 0.1 µm ceramic graded permeability membranes that were fed with 3 different milks: SM, MPC85 that had been standardized to the protein content of SM with reverse osmosis (RO) water (MPC), and MPC85 that had been standardized to the protein and lactose contents of SM with RO water and lactose monohydrate (MPC+L). Retentate and permeate were continuously recycled to the feed tank. The LF for each feed was determined by increasing flux once per h from 55 kg·m⁻²·h⁻¹ until flux did not increase with increasing transmembrane pressure. Temperature, pressure drop across the membrane length, and protein concentration in the retentate recirculation loop were maintained at 50°C, 220 kPa, and 8.77 ± 0.2%, respectively. Experiments were replicated 3 times and the Proc GLM procedure of SAS was used for statistical analysis. The LF of SM (91 kg·m⁻²·h⁻¹) was lower (P < 0.05) than the LF of MPC+L (124 kg·m⁻²·h⁻¹) due to the role of calcium in fouling. The LF of MPC+L was lower (P < 0.05) than the LF of MPC (137 kg·m⁻²·h⁻¹) due to the higher viscosity contributed by lactose. Permeates produced from the MPC and MPC+L contained more (P < 0.05) protein than the SM permeate due to the transfer of micellar casein into the reduced-calcium sera of the MPC and MPC+L. SPR was calculated by dividing true protein in the permeate by SP in the permeate portion of the feed to describe the ease of SP passage through the membrane. After accounting for the nonmicellar casein with SDS-PAGE, no differences (P > 0.05) in SPR were detected among the 3 feeds below the LF. As the fluxes approached the LF, SPR decreased (P < 0.05) due to fouling. Feeding a MF system with MPC instead of SM will reduce the required membrane surface area, but the permeate protein composition will be different.