Although there are numerous inorganic model systems that are readily presented as undergraduate laboratory experiments in bioinorganic chemistry, there are few examples that explore the inorganic chemistry of actual biological molecules. We present a laboratory experiment using the oxygen-binding protein myoglobin that can be easily incorporated into an undergraduate laboratory course. This highly intuitive experiment in which ligand-binding reactions of carbon monoxide and dioxygen are monitored using scanning visible absorption spectroscopy provides the opportunity to investigate the relative binding strengths of O[subscript 2] and CO to the reduced iron porphyrin center. This process represents an important first step in the mechanism of respiration and offers a convenient entry point into several interesting topics for discussion including, for example, the role of metals in biology; ligand binding and redox states in metalloporphyrins; the application of spectroscopic probes to the study of kinetics and mechanism; protein dynamics; and structure-function relationships. (Contains 3 figures and 3 notes.)