A slab gel cassette is formed from generally flat plates welded together along their borders with a gel space between the plates that is open at the top and bottom edges of the plates to expose the opposing ends of a gel in the gel space to buffer solutions in which electrodes are immersed. The plates are shaped such that the weld line does not intersect with the bottom opening of the gel space. Sealing of the bottom of the gel space, which is necessary for gel casting, is achieved by placing a sealing strip over the bottom opening with no danger of leakage of the gel solution along a weld line.