A rapid and sensitive fluorescence assay for radiation-induced DNA damage is reported. Changes in temperature-induced strand separation in both calf thymus DNAand plasmid DNA (puc 19 plasmid from Escherichiacoli) were measured after exposure to low doses ofradiation. Exposures of between 0.004 and 1 Gy weremeasured with doses as low as 0.008 Gy yielding significant responses. The double-strand, sensitive dye PicoGreenwas used as an indicator of DNA denaturation. Calibrationplots indicate that fluorescence changes correspondingto amounts as low as 1 ng of double stranded DNA (106copies for plasmid puc 19) are detected by this method.