The Mass Spectrometry Laboratory of the University of Texas Health Science Center at San Antonio functions as a multi-user support service for the Health Science Center and the Southwest Research Consortium. Currently, there are two Hewlett-Packard 5982 quadrupole mass spectrometers, a Finnigan-MAT magnetic sector instrument and a Finnigan-MAT 4615 quadrupole system. One of the Hewlett-Packard instruments has been modified for thermospray HPLC-MS and the other is utilized for quantification of catechol metabolites in psychiatric patients. The magnetic sector mass spectrometer is used primarily for FAB and some non-routine GC/MS analyses, while the 4615 handles the majority of the direct insertion probe and GC/MS measurements. These instruments have been extremely valuable in providing mass spectrometric analyses for a wide variety of compounds from many different areas of biomedical research. One area that is deficient, however, is measurement of peptides with a molecular weight over 2,000 daltons. There are two peptide synthesis facilities at this institution and one at Southwest Foundation for Biomedical Research (a member of the Southwest Research Consortium). The FAB capabilities of the Finnigan-MAT 212 have been very satisfactory for molecular weight determination of synthetic peptides for a few investigators, because of their products are suitable for the mass range of the instrument. However, the majority of the peptides being synthesized here are too large to be analyzed on the 212. It has become increasing clear to investigators in peptide research that problems often occur during synthesis that prevent the expected product from being obtained. But in many cases, difficulties such as incomplete removal of amino acid protecting groups or chemical alterations caused by cleavage procedures go undetected by techniques such as amino acid analysis or HPLC. Mass spectrometry is the only way to unambiguously determine if a synthesis is correct. Since the synthetic peptides are used in varied types of research, such as for antisera production, generation of vaccines and evaluation of enzyme structure, inhibition and receptor binding, it is of critical importance that the peptide composition be verified. Otherwise, the results of countless subsequent experiments could be jeopardized. Furthermore, there are measurements involving photolabelled peptides and glycolipid components of bacterial membranes that cannot be accomplished with our current instrumentation. After evaluation of the benefits of various types of mass spectrometers, the BIOION 20 plasma desorption mass spectrometer (PDMS) has emerged as the best choice for our needs in terms of mass range, analytical power, ease of use, precision, maintenance and cost. Availability of the PDMS would be of major importance for analysis of synthetic peptides and for assistance in protein sequencing, molecular weight determination of newly isolated peptides and analysis of other substances such as glycolipids. Acquisition of the PDMS is crucial to facilitate further progress in numerous ongoing investigations.