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Fluorescent perylene colloids in the 80-90 nm size range have been prepared by the reprecipitation method. These nanoparticles were modified by cetyltrimethylammonium bromide (CTAB) which inhibited their growth. The nanoparticles also readily interacted with DNA. The fluorescence emission was measured at lambda(ex)/ lambda(em)=400/565 nm. The fluorescence decrease of colloid-CTAB in aqueous solution was measured in the presence of nucleic acids. Under the optimum conditions, the ratio of fluorescence intensity in the absence and presence of nucleic acids was proportional to the concentration of nucleic acids over the range 0.02-5.1 micro g mL(-1) for FS (fish sperm) DNA or CT (calf thymus) DNA. The detection limits were 0.01 micro g mL(-1) for FS DNA and 0.012 micro g mL(-1) for CT DNA, respectively. Based on this approach, a new quantitative method for DNA assay is presented in this paper.