In recent years, mass spectrometry has become the method of choice for identifying small amounts of gel separated proteins. Using high mass accuracy peptide mass mapping followed if necessary by nanoelectrospray sequencing, most mammalian proteins can now be identified quickly and sensitively either in amino acid or in EST sequence databases. These methods are illustrated here using an ongoing project in the author's laboratory, a mass spectrometric screen for new mouse brain receptors and their interaction partners.