A comparison of two automatic peak detection algorithms is presented. One algorithm comes with the Voyager 5 Data Explorer^TM program, the other is a new algorithm called Pepex® (short for PEptide Peak Extractor) from BioBridge Computing. The peak sets selected with both tools have been compared, against each other and against manual peak selections, on a large set of mass spectra obtained after tryptic in gel digestion of 2D-gel samples from human fetal fibroblasts. It is shown how much variation there is in peak sets, both when selected by human operators and when selected by automatic peak detection algorithms. This variation has been used as an advantage to gain significantly better protein identification results, using the Pepex tool, than what an experienced mass spectroscopist has achieved on the same data. The strongest improvement has been observed in weak spectra, where the signal peak intensities are low.