A computer program for personal computers has been developed, which enables the user of interpret fragmentation processes in electron impact mass spectrometry more easily. Starting with a low resolution mass spectrum and a selected chemical structure, fragments of the molecule which are related to the spectral data can be created interactively. Some frequently occurring fragmentation reactions can be performed automatically. An included editor allows the user to draw chemical structures and to create or modify mass spectra. Fragments of a molecule can be displayed as a tree. One aim of the program is to facilitate learning of fragmentation reactions. Furthermore, the program serves as a tool for verifying fragmentation mechanisms proposed and for checking presumptive chemical structure formulas of unidentified compounds.