In pre-cast slab gel cassettes, the formation of pathways in which proteins can migrate between the gel and the walls of the cassette to form shadow bands is avoided by including a nonionic amphiphilic polymer in the monomer solution from which the gel is formed and casting the gel with the polymer included. The nonionic amphiphilic polymer also prevents the resulting gel from sticking to the walls when the gel is to be removed from the cassette after electrophoresis.