Following tryptophan fluorescence of the galactose-binding during dissociation of the ligand it has been found that glucose dissociates with a half life of less than 5 s. Similarly, fast dissociation was also observed by following release of radioactively labelled glucose from Sepharose-coupled galactose-binding protein upon dilution. Accordingly, a previous claim that the galactose-binding protein contains glucose as a non-dissociable "hidden ligand" [G. Richarme and A. Kepes (1974) Eur. J. Biochem. 45, 127-133] has to be reinterpreted