Abstract The dye 8-anilino-1-naphthalenesulfonic acid reacts with whole calf thymus histone to form a fluorescent complex having an excitation maximum at 375 mμ and an emission maximum at 500 mμ. The degree of reaction, and hence fluorescence, is enhanced at low pH values. At pH 2, maximum fluorescence is obtained in the region of 1.5 μg dye to 1.0 μg histone. Dye concentrations in excess of 250 μg/ml result in serious self-extinction. Dye-to-protein ratios considerably lower than 1.5:1 result in incomplete saturation of protein-binding sites, resulting in lowered fluorescence intensity. When the proper ratio is maintained, fluorescence intensity shows a linear relationship to protein concentration over the range of 1–300 μg histone/ml of sample. The fluorescence assay may be used for quantitation purposes within this range and for qualitative purposes with higher protein concentrations.