The complexation reaction between the amphiphilic peptide antibiotic polymyxin B and naturally occurring cyclodextrins, used as potential drug carriers, was quantitatively evaluated from surface tension measurements at various drug concentrations, The association constant, K-a, of polymyxin B:beta-cyclodextrin inclusion complex formation of 1:1 stoichiometry was determined from the change in the drug interfacial activity upon the addition of beta-cyclodextrin at the excess solution concentration (10(-3) M). The obtained K-a value is discussed in terms of molecular matching of the host cyclodextrin cavity and the guest drug molecule.