A new method for determining the zeta-potential of a membrane or other porous media is proposed in which an alternating electric field is applied in the measurement of the electroosmotic flux. Determination of the apparent zeta-potential of commercial microfiltration membranes was conducted under different alternating frequencies. An increase in the magnitude of zeta-potential is obtained as a result of increased alternating frequency. A dramatic change of the apparent zeta-potential was observed when the membrane was soaked for 24 hours in a solution containing bovine serum albumin (BSA). The magnitude of the zeta-potential approached zero at pH 4.9, which indicated the adsorption of BSA on the membrane surface. When the membrane sample was soaked for 24 hours in a BSA solution containing PEG 4000, a substantial increase in the magnitude of the zeta-potential compared to that in the BSA solution was obtained. In some cases the apparent zeta-potential obtained in the BSA-PEG 4000 solution was up to that obtained in the BSA-free solution. This demonstrated the shielding function of PEG 4000 as described elsewhere. The zeta-potential was shown to be a sensitive indicator of membrane fouling caused by protein adsorption.