To study the mutagenicity and carcinogenicity of Nabumetone, we conducted Ames test (TA97, TA98, TA100, TA102), micronucleus test(MN) in mice marrow, chromosomal aberration assay(CA) in CHL cells in vitro, CA in germ cells from testes of mice, and cell transformation test of Syrian hamster embryo(SHE) cells. The maximum concentration was 500 micrograms/plate in Ames test with and without S9 mix. The mice were treated orally(gavage) daily for 4 days in 3 doses in which the maximum dose was 60% LD50 and sampled at the 5th day in MN. The maximum concentration was the dose that the growth of 50% of cells was inhibited in CA of CHL. Cells were harvested after recultured for 18 hours in fresh medium after treatment for 6 hours in the test with S9 mix, and after treatment for 24 or 48 hours in the without S9 mix. The mice were treated orally(gavage) daily for 5 days in 3 doses in which the maximum dose was 1/4 LD50 and sampled at the 6th day in CA of germ cells from testes of mice. 2 micrograms/ml was chosen as the maximum concentration in the cell transformation test of SHE cells, and result was observed after treatment for 9 days, All the tests obtained the same negative result as that reported by other investigators.