A new apparatus was developed where streaming potentials and permeate fluxes of membranes could be measured simultaneously. In this way the effect of addition of a protein, bovine serum albumin, on the potential and flux could also be studied. On addition of protein the calculated zeta potential of the membrane changed so as to be close to the potential of the protein at that pH. At very low or high pH, where the protein and the membrane had the same sign of charge, adsorption decreased and the potential of the membrane did not change fully to that of the protein. The point of zero charge of the protein-covered membrane was slightly higher than the isoelectric point of the protein.