Fluorescence quenching assays are often used to measure dissociation constants that quantify the binding affinity between small molecules and proteins. In an upper-division undergraduate laboratory course, where students work on projects using a guided inquiry-based approach, a binding titration experiment at physiological pH is performed to analyze the interaction between the drug norfloxacin and the protein trypsin. The resulting nonlinear binding data are fitted using a scientific data analysis program to determine the dissociation constant (K[subscript d] value) describing this interaction. The experiment is especially of interest to students who plan to pursue careers in health-related aspects of biochemistry.