A method for the determination of nucleic acids with methylene blue(MB) as a fluorescence probe was developed, based on the quenching of the fluorescence of MB in the presence of nucleic acids. The calibration graph was linear in the range of 0–40 μg/ml for CT DNA and 0–55 μg/ml for yeast RNA, the limit of detection was 28 ng/ml for CT DNA and 82 ng/ml for yeast RNA. CT DNA could be determined in the presence of 30%(w/w) yeast RNA and yeast RNA was determined when the content of CT DNA in synthetic samples was below 5%(w/w).