Abstract : 3-Nitro-1,2,4-triazol-5-one (NTO, 99.6% pure) was tested for its potentials to induce chromosome aberrations in cultured Chinese Hamster Ovary (CHO) cells with and without metabolic activation according to OECD TG 473 in compliance with Good Laboratory Practice. NTO was found negative at concentrations of 5, 50, 100, 500, 1000, 2500, and 5000 g/mL in both with and without activation. A confirmatory chromosome aberration assay was performed without activation also showed negative.