Peptide fingerprint mass mapping is widely used for initial identification of proteins separated by gel electrophoresis. This method involves the enzymatic in-gel digestion of proteins to generate peptides, followed by mass measurement of the cleaved peptides. The experimental mass values are then compared with theoretical values from protein databases, calculated using the cleavage specificity of the enzyme. This unit describes the use of the search program MS-Fit to identify the closest match or matches.