A simple tluorometric assay for N-hydroxyarylamine O-acetyltransferase is described and compared with a nucleic acid-binding assay. The assay method is based on the finding that the highly mutagenic 2-hydroxyamino-6-methyldipyrido[ 1,2-a:3',2'-dlimidazole (N-OH-Glu-P-I) was reduced to the corresponding amine, 2-amino-6-methyldipyrido[ I ,2-a:3',2'-dlimidazole (Glu-P-I), through N-acetoxy-Glu-P-l as the reactive intermediate in the presence of N-hydroxyarylamine O-acetyhransferase, acetyl-CoA, and a sulfhydryl compound. The formation of Glu-P-I was determined by its characteristic fluorescence intensity at 445 nm with excitation wavelength at 376 nm. The reductive reaction was inhibited by the addition of tRNA, DNA, and poly(G), to which the enzymatic product, N-acetoxy Glu-P-I, bound effectively due to its electrophilic nature. Since the fluorometric assay for the O-acetyltransferase is rapid, simple, and sensitive as compared with the nucleic acid-binding method using radioisotope-labeled N-hydroxyarylamine, this method is applicable to the general assay for the formation of reactive N-acetoxy-Glu-P-l.