An electrophoresis gel is prepared with indicia incorporated into the gel itself rather than on a label or backing that is adhered to the gel. According to one method, the indicia are applied by first coating a solid surface that will be used as one internal surface of a gel mold with a solution of a water-permeable polymer, allowing the coating to dry, imprinting indicia in ink on the dry coating, and then forming the gel over the coating. In a variation on this method, the indicia are applied to a sheet which is then placed against the mold surface, the sheet being either a liner that is separable from the gel yet coated with the polymer or a sheet of the polymer itself. Alternatively, the indicia are printed directly on the mold surface without first coating the surface. This is achieved by using as a printing composition a mixture of an ink and a solution of a water-permeable polymer, then forming the gel over the imprinted indicia.