Abstract: Apparatus for vertical slab gel electrophoresis has upper and lower tanks having open tops. The tanks are supported in stepped relation to each other by a vertical support plate which forms a front wall of one tank and a rear wall of the other. At least part of the wall of the upper tank formed by the support plate has a top edge which is lower than the other top edges of the tank. A substantially open-ended chamber is formed by clamping a plastic plate to a glass plate with strip members between side edges of the plates. The plastic plate is held against the spport plate for reinforcing the rigidity of the plastic plate with the support plate. A portion of the top edge of the plastic plate is lower than the top edge of the glass plate and intermediate the lower edge portion of the wall of the upper tank formed by the support plate and the other top edges of the upper tank.