In this discovery-oriented laboratory experiment, students use solid-phase synthesis techniques to construct a dipeptide containing an unknown amino acid. Following synthesis and cleavage from the polymeric support, electrospray ionization-mass spectrometry is employed to identify the unknown amino acid that was used in the peptide coupling. This experiment, performed in two, 3-h laboratory periods, provides an opportunity for students to learn the technique of solid-phase peptide synthesis and to critically evaluate data that they generate in the laboratory. The experiment is suitable for second-semester organic chemistry students as they learn methods for the formation of amide bonds or upper-level biochemistry students as they learn about the primary structures of proteins.