The interaction between chlorpromazine (CPZ) and riboflavin binding protein (RBP) was. studied by the spectrophotometric and spectrofluorometric methods.CPZ combined with RBP in a I: 1 molar ratio and the dissociation constant was determined to be 3.8 x 1O-5M at pH 7, 25°C. F1avins such as riboflavin (Rf) and lumiflavin were released from the complexes with RBP by the addition of CPZ. The mechanism of flavin-release was mainly competitive binding between CPZ and a isoalloxazine ring of flavins to RBP.The mechanism of some biological phenomena induced by CPZ was discussed based on the flavin-CPZ interaction.Abbreviations: CPZ, chlorpromazine; Rf, riboflavin; RBP, riboflavin binding protein; FAD, flavin adenine dinucleotide.