The contribution of mass spectrometry to the solution of problems in protein biochemistry was limited until the development of methods of ionization that do not require derivatization or prior vaporization of the sample. Fast atom bombardment (FAB), introduced by Barber et al. in 1981 (1), is one of the most important of these methods, and has been widely applied in the peptide and protein field. In the FAB experiment (Fig. 1), the sample is dissolved in a liquid of low vapor pressure, often glycerol or thioglycerol (“the matrix”), and is bombarded by a beam of energetic particles, such as xenon atoms that sputter sample molecules from the surface layers of the matrix into the mass spectrometer vacuum. Proton or other cation attachment produces abundant (positive) ions characteristic of the sample's molecular mass. A proportion of these molecular ions dissociate, producing structurally informative fragments that are generally less intense than the molecular ions, since the ionization process imparts relatively little excess energy. Negatively charged ions are also generated, and spectra may be recorded in either mode by appropriate selection of the polarity of the ion extraction voltages. At low-mass FAB, spectra are generally dominated by signals attributable to ionization of the matrix. The background of “chemical noise” extending to high mass, which gives FAB spectra their characteristic peak-at-every-mass appearance, is probably attributable to direct hits on sample and matrix molecules by the bombarding species. Figure 2 shows a typical FAB spectrum of the cyclic heptapeptide microcystin-LR, obtained from the cyanobacterium Microcystis aeruginosa.