This paper describes a method for the detection of single-base mismatches using DNA microarrays in a format that does not require labeling of the sample ("target") DNA. The method is based on disrupting fluorescence energy transfer (FRET) between a fluorophore attached to an immobilized DNA strand ("probe") and a quencher-containing sequence that is complementary except for an artificial mismatch (e.g. 5-nitroindole, 3-nitropyrole, or abasic site) at the site of interrogation. As the displacement of the FRET acceptor and hybridization of the unlabeled probe are bimolecular, the term "bimolecular beacons" is used to describe this approach. The analysis of a mismatch was based on differences in the amount of disruption in FRET upon hybridization of perfectly matched DNA targets and those containing single-base mismatches. Using this method and an oligonucleotide model system, A/C single-base mismatches were successfully discriminated at levels greater than that observed using surface-immobilized molecular beacons. The amount of discrimination was dependent on the identity of the artificial mismatch; greater discrimination was observed with 5-nitroindole (a "universal" base) than with an abasic site. G/T mismatches, considered to be particularly difficult to detect, were also successfully discriminated when quencher sequences containing 5-nitroindole were used.