A sensitive assay for detecting double-stranded (ds) DNA in solution is described. This assay employs a new dye, PicoGreen dsDNA quantitation reagent, which becomes intensely fluorescent upon binding nucleic acids. The brightness of this reagent is due to its high quantum yield (approximately 0.5, bound to ds calf thymus DNA) and large molar extinction coefficient (approximately 70,000 cm-1 M-1). The fluorescence enhancement of this dye upon binding dsDNA is > 1000-fold, with excitation and emission maxima near those of fluorescein. Unlike Hoechst 33258, PicoGreen reagent fluorescence intensity was the same upon binding to poly(dA).poly(dT) and poly(dG).poly(dC) homopolymers. The PicoGreen assay allowed the detection of 25 pg/ml dsDNA, surpassing the sensitivity achieved with Hoechst 33258 by 400-fold. The linear concentration range for DNA quantitation extended over four orders of magnitude-25 pg/ml to 1 microgram/ml-with a single dye concentration. Assay linearity was maintained even in the presence of salts, proteins, poly(ethylene glycol), urea, chloroform, ethanol, and agarose; some ionic detergents and heparin interfered. Linear DNAs yielded slightly brighter signals than supercoiled plasmids. Finally, the assay showed greater dsDNA:RNA selectivity than Hoechst 33258 in low ionic strength buffer and better dsDNA:single-stranded DNA selectivity in 1 M NaCl.