The electrophoretic mobility of proteins in membrane pores has been investigated experimentally. When the size of the protein is small relative to the pore size, the protein mobility is identical to the free protein mobility. As the pore radius approaches the protein radius the mobility of the protein is significantly reduced. This phenomenon has been explained in terms of electrokinetic theory. Using a method of reflections, and taking into account the effect of the back-flow, an approximation has been developed for the average mobility in a closed system of a spherical particle moving under electrophoresis parallel to the axis of a cylindrical pore. This approximation assumes that the surface potential of the particle is low, and is valid for arbitrary double layer thickness relative to particle size, provided that there is minimal overlap between the double layers at the pore surface and around the particle. It is also predicted that when the protein and the membrane have surface potentials of the same sign, there can be a significant increase in protein mobility for medium-sized pores.