A horizontal electrophoresis assembly is described which is designed to utilize very thin gels of polyacrylamide. The assembly allows for water cooling of the gel during electrophoresis so that higher voltages can be applied to the gel without polymer degradation. The gel mold is made from a set of glass plates which are clamped into place defining a gel mold between them. In one embodiment, the site of field introduction is separated from the site of sample introduction so that the electric field is generally linear at the region of sample introduction.