We describe an approach for fractionating complex protein samples prior to two-dimensional gel electrophoresis using reversed-phase high-performance liquid chromatography. Whole lysates of cells and tissue were prefractionated by reversed-phase chromatography and elution with a five-step gradient of increasing acetonitrile concentrations. The proteins obtained at each step were subsequently separated by high-resolution two-dimensional gel electrophoresis (2-DE). The reproducibility of this prefractionation technique proved to be optimal for comparing 2-DE gels from two different cell states. In addition, this method is suitable for enriching low-abundance proteins barely detectable by silver staining to amounts that can be detected by Coomassie blue and further analyzed by mass spectrometry.