All nucleotides examined (AMP, GMP, TMP and CMP) quench the fluorescence or 10methylacridinium chloride (10-MEAC). The fluorescence spectrum of 10-MEAC-nucleotide system is identical with that of 10-MEAC itself, and the fluorescence decay kinetics follow a single-exponential decay law. The dependence of fluorescence quantum yields and fluorescence lifetimes upon the concentration of nucleotides indicates that the fluorescence of 10-MEAC is greatly quenched in both dynamic and static processes by nucleotides. The quenching constants increase in the order: A M P c GMP > TMP c CMP. The results of 10-MEAC are compared with those of other acridine dyes (proflavine, 9-aminoacridine and acridine orange).