A new method for determining ζ- potential of membrane in solution was developed on the basis of measuring electroosmosis flux in an alternating electric field. This method was effective in dealing with the concentration polarization that occurred during the measurement. The function of polyethylene gly-col 4000 to shield the interaction between microfiltration membranes and protein was examined at different pH of the solution. It was shown that ζ- potential could serve as an index of membrane fouling and be used in the study of the nature of membrane fouling.