Abstract Fibrinopeptide B ( M r 1552.58) was employed as a calibration compound for matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) post-source decay (PSD) fragment ion analysis in the negative mode. Experiments were performed by using both continuous and delayed extraction, with the maximum reflectron voltages being 30 and 21 kV, respectively. For comparison, a common positive ion PSD calibrant, ACTH(18–39) ( M r 2466.7), was also employed with positive ion calibration constants being applied to negative ion spectra. Using fibrinopeptide B as the calibrant, the negative ion PSD results for angiotensin II ( M r 1046.2), renin substrate tetradecapeptide (horse) ( M r 1759.0), and the custom-synthesized peptide (K 2G 4) 2 ( M r 987.1) showed a factor of 1.5–2 improvement in absolute mass accuracy. Typical absolute mass-to-charge ratio accuracies were within ±1 Thomson and were achieved even when the peptide being analyzed was more massive than fibrinopeptide B. In addition, both calibrants showed increased accuracy when experiments were conducted in the delayed extraction mode. Other advantages of using fibrinopeptide B are its moderate cost and the ability to perform calibration and sample analysis for negative ion PSD under the same instrumental conditions.