The surface behavior and the critical micelle concentrations of the penicillin sodium nafcillin monohydrate in aqueous electrolyte solution (0.00-0.40 mol kg(-1) NaCl) at 303.15 K were determined by surface tension measurements. From the magnitude of the surface properties and using the values of the standard Gibbs energy of micellization of this penicillin, the standard Gibbs energies of adsorption of this drug were calculated at each electrolyte concentration.