We describe a quantitative assay of dsDNA based on real-time PCR measurement of fluorescence due to the interaction of PicoGreen dye with dsDNA. An aliquot of 1 to 5 ml of the sample is mixed with 45 ml of diluted PicoGreen reagent within an optical PCR tube. This is placed into the real-time apparatus set to read SYBR Green I dye at the end of three cycles of 94 degrees C for 30 s and 65 degrees C for 30 s. The averaged fluorescence value is converted into DNA amount using a calibration curve prepared with lambda-DNA standard. The calibration curve has a dynamic linear range from 0.20 to 50 ng and a standard deviation variability below 5.0%. In conclusion, this method allows reliable determinations on minimal amounts of DNA from biological samples and PCR products in clinical applications of molecular biology.