Streaming potential and conductivity measurements were performed on porous plugs in a number of indifferent electrolytes. The plugs were made of monodisperse spherical polystyrene sulfate latex with fixed surface charge density. The observed surface conductivity could be interpreted with the classical Bikerman expression extended with the contribution from conduction behind the shear plane. It was shown that electroosmosis contributes positively, while the depletion of co-ions in the double layer contributes negatively to the surface conductivity. It was further demonstrated that a significant part of the conduction is located behind the slip plane. The ion mobility of the indifferent counterions behind, as well as beyond, the shear plane is close to the bulk mobility. In the correct determination of zeta-potentials in the low electrolyte concentration range, it is necessary to take surface conduction behind the plane of shear into account. When this is done, the maximum in the curve of the zeta-potential versus electrolyte concentration disappears.