Biological mass spectrometry is an important analytical technique in drug discovery, proteomics, and research at the biology-chemistry interface. Currently, few hands-on opportunities exist for undergraduate students to learn about this technique. With the 2002 Nobel Prize being awarded, in part, for the development of biological mass spectrometry, more academic institutions will hopefully incorporate these techniques into their curricula. We present the first in a series of experiments designed to introduce students to the analysis of biological molecules using mass spectrometry. In this experiment, students analyze a medium-sized protein using electrospray ionization and identify it based on molecular weight. Since electrospray ionization produces multiply charged ions and since isotopic distributions cannot be easily observed for highly charged ions, a mathematical treatment of sequentially charged ions is used to determine the charge state on the ion signals and thus calculate the molecular weight. Averages of several differently-charged ion signals from the same analyte are used to obtain an average molecular weight. We have included several representative protein spectra, suitable for classroom use, in the Supplemental Material. (Contains 1 table, 1 figure, and 2 notes.)