The development of a high-performance liquid chromatography (HPLC)/fast atom bombardment (FAB) interface and the subsequent commercial availability of such systems has facilitated the routine application of the technique to problems in pharmaceutical research and development. Although many products are amenable to F A 9 analysis and hence LCFAB, the greatest benefit of the interface has been in the field of peptide and protein analysis. It has been found that, even with post-column matrix addition, chromatographic resolution is maintained and, by plotting mass chromatograms, the resolution may be greater than that achieved by the less specific UV detector. As only 1% matrix is required in the final eluent, the system is stable for extended periods and has been used for 3 h L C F A B experiments or used continuously for multiple analyses over 8 h periods. In addition to the acquisition of relative molecular mass information, the constant background can be completely subtracted to yield structurally significant fragment ions which may allow sequencing of components from the single L C F A B experiment. Applications of L C F A B to date include the characterization of lys(78)-plasminogen by the on-line analysis of complex mixtures of peptides resulting from the various enzymatic digests.