An alumina microfiltration membrane of 0.2 mu m pore diameter was used for the concentration of whey proteins in solutions of NaCl (0.08 M) and whey UF permeate at different pH, in order to study the effect of the physicochemical environment on the hydraulic resistance and the rejection of whey proteins during microfiltration. Similar filtration behaviour was found between the systems in 0.08 M NaCl and whey UF permeate. Lowering the pH of the protein solutions caused an important increase in total hydraulic resistance particularly for the reversible and irreversible partial hydraulic resistances. The rejection of proteins (greater than or equal to 0.40) was only slightly affected by differences in solvents and pH. It is believed that protein deposition in the pores under the influence of shear and electric charges of both membrane and proteins is responsible for the hydraulic resistances measured.