GEL electrophoresis in discontinuous buffer systems1 is a valuable method of examining dilute protein solutions. By selecting pH conditions it is possible to concentrate and then separate either cationic or anionic proteins, but only in separate analyses. Thus, the tris-glycine system can be used, in which the running pH is 9.5 and most proteins migrate as anions1. The usual cationic proteins will either not migrate or will move backwards as a large zone equal in size to the origin. Conversely, an acidic system, such as β-alanine-acetate, may be chosen in which cationic proteins migrate at pH 4.0 (ref. 2), but in, this case the anionic proteins are lost to further analysis. Apart from the loss of time and research material necessitated by separate analyses, there is an unavoidable ambiguity about the direction of migration of a particular protein, because it is quite possible for the same protein to migrate as an anion at pH. 9.5 and as a cation at pH. 4.0. Clearly, a single system allowing the simultaneous separation of oppositely charged proteins would be desirable. To this end the method described couples two discontinuous buffer systems which provide concentrating and running conditions for both cationic and anionic proteins in the same sample.