Membrane protein complexes such as the reaction center complexes of oxygenic photosynthesis or the complex I of mitochondira are composed of many subunit polypeptides. To analyze their polypeptide compositions by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), a wide range of molecular sizes has to be resolved, especially in the low molecular mass range. We have improved the traditional Tris/HCl buffer systems adopting a Tris/2-(N-morpholino)ethanesulfonic acid (MES) buffer system containing 6 M urea. This gel system was used with an 18-24% acrylamide gradient for the separation of polypeptides with molecular masses from below 5 kDa to over 100 kDa. This buffer system can also be applied to the usual uniform concentration of acrylamide gel and also to minislab gels.