SDS-PAGE is a simple, rapid technique that has many uses in biochemistry and is readily adaptable to the undergraduate laboratory. It is, however, a technique prone to several types of procedural pitfalls. This article describes the use of SDS-PAGE to determine the subunit molecular mass and composition of yeast alcohol dehydrogenase employing state-of-the-art methods that produce accurate results. Students apply precise amounts of protein to the gel to determine the optimal amount required for accurate measurements. Precision Plus Protein standards are used to produce a standard curve. Students calculate the true subunit molecular mass of the protein using information from the ExPASy protein knowledgebase. They use published data from gel filtration chromatography to determine the molecular mass of the native protein and use this information and their experimental data to determine the subunit stoichiometry of the protein. (Contains 1 table and 3 figures.)