A novel fluorimetric method has been developed for rapid determination of DNA and RNA with hypocrellin A (HA) as a fluorescence probe, based on the fluorescence enhancement of HA in the presence of DNA or RNA. Maximum fluorescence is produced in the pH range 3.4–4.0, with maximum excitation and emission wavelengths at 470 and 600 nm, respectively. Under optimal conditions, the calibration graphs are linear over the range 0–200.0 ng cm ^{- 3} for calf thymus DNA and 13.0–200.0 ng cm ^{- 3} for yeast RNA, respectively. The corresponding detection limits are 5.0 ng cm ^{- 3} for calf thymus DNA and 13.0 ng cm ^{- 3} for yeast RNA. The relative standard deviation of six replicate measurements is 4.5% for 100 ng cm ^{- 3} calf thymus DNA. DNA could be determined in the presence of 20% m/m yeast RNA. The mechanism for the binding of HA to DNA is also studied.