Mass spectrometry has become a routine analytical tool in the undergraduate curriculum in the form of GC-MS. While relatively few undergraduate programs have incorporated biological mass spectrometry into their programs, the importance of these techniques, as demonstrated by their recognition with the 2002 Nobel Prize, will hopefully lead to increased opportunities for undergraduate students to experience the power of these techniques. We present a laboratory experiment that allows students to determine the molecular mass of a peptide based on the spectrum obtained from analysis using electrospray ionization mass spectrometry (ESI-MS). Since ESI-MS often produces multiply charged ions, students must use isotopic patterns to first determine the charge state on the ion signals prior to calculating the molecular mass and thus the identity of the peptide. If the charge state determination is not done correctly, large mass errors larger can occur, while students routinely observe mass errors of approximately 0.01% when the analysis is done correctly. (Contains 1 table and 2 figures.) [For Part 1, see EJ820900.]