Sequential thin slicing of one-dimensional electrophoresis gels followed by slice-by-slice mass spectrometry to allow protein identification was used to produce a proteomic map for cytochromes P450. Parallel MALDI-TOF-MS and LC-MS/MS analyses were performed. Combination of the two MS methods increased the quality of protein identification. We have proposed an efficient approach to obtain a comprehensive profile of drug-metabolizing enzymes in the liver that can be used to differentiate between polymorphic variants of cytochromes P450.