The binding of dantrolene sodium to human serum albumin was studied by fluorescence quenching and difference spectrophotometry. The association constant was calculated from each method of measurement and was large. This binding affinity may be of importance in the clinical setting, since competitive displacement of anionic drug by concurrently administered agents can occur. Consequently, the displacement of dantrolene from albumin was examined with a wide range of drugs. To gain insight into the characteristics of drug-albumin binding, the interaction of drug with cationic, anionic, and nonionic surfactants was also studied. Additions of drug to solutions of either the anionic or nonionic surfactant failed to result in a perturbation with the difference spectral technique. However, dantrolene added to the cationic resin produced a difference spectrum analogous to that observed with the drug-protein interaction.