A number of recombinant proteins are being produced as pharmaceuticals with annual sales of U.S. $9 billion in 1994 [1,2]. Therapeutic applications of these biopharmaceuticals include stimulation of cell growth, regulation of the immune system and fibrinolysis, to name a few. These advances of recombinant proteins toward the marketplace was made possible by the development of efficient and reproducible production and purification systems that permit manufacture of these complex molecules in large amounts with consistent quality suitable for human use. Control of the quality of biopharmaceuticals is crucial not only during largescale manufacturing but also during the research and development phase, as impurities and contaminants have to be characterized and reduced to acceptable levels.