The inclusion of 4-hydroxybenzoic methyl ester (methyl paraben) in alpha- and beta-cyclodextrin has been studied by fluorescence spectroscopy. Qualitative evidence for host-guest binding comes from a study of the spectroscopic behavior of methyl paraben included in alpha-cyclodextrin. To describe quantitatively the complex formation an association constant of 458 +/- 34.8 M(-1) at 21 degrees C was obtained. The inclusion complex of methyl paraben in alpha-cyclodextrin permits quantitation in the range 0.04-3 pg/ml. The method has been applied to determine methyl paraben in non alcoholic beverages.