Electrophoresis is a valuable tool for biochemists, yet this technique is often not included in biochemistry laboratory curricula owing to time constraints or lack of equipment. Protein structure is also a topic of interest in many disciplines, yet most undergraduate lab experiments focus only on primary structure. In this experiment, students use sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and beta-mercaptoethanol (betaME) to determine preliminary information about protein quaternary structure. This experiment gives students a better understanding of protein subunit composition and the forces responsible for the stability of quaternary structure. The experiment can be accomplished in one three-hour laboratory period. During the 2002-2003 year, students examined four proteins with varying subunit compositions: carbonic anhydrase, hemoglobin, glyceraldehyde 3-phosphate dehydrogenase, and superoxide dismutase. By comparing data in the presence and absence of betaME, students correctly determined that superoxide dismutase was the only protein examined in which an intermolecular disulfide bond helps stabilize quaternary structure; the other proteins have only noncovalent interactions between subunits. Student assessment surveys indicate improved understanding of gel electrophoresis and protein subunit composition. Students agreed that there was adequate time to complete this experiment and recommend its inclusion in future labs.