In vitro antimutagenic activity of methanol extract from brown rice and its physico-chemical characteristics were investigated using Salmonella typhimurium reversion assay and SOS chromoteat. Methanol extracts of brown rice were not mutagenic compared with direct and indirect mutagenicities of 4NQO (4-nitraquinoline oxide), 2NF(2-nitrofluorene), Trp-p-1(3-Amino-1,4-dimethyl-SH-pyrido-[4,3-b]indole), and Trp-p-2(3-Amino-l-methyl-SH-pyrido[4,3-b] indole). Antimutagenic activity against the indirect mutagenicitiea induced by Trp-p-1, Trp-p-2 and AFB₁ (aflatoxin B₁) was found in methanol extract. Even though antimutagenic activity showed dose-dependent, it remained constant at inhibition rate ranging 60%-90% when the concentration was above 3mg/plate in the S. typhimurium reversion assay and 0.2-0.6 mg/assay in the SOS chromotest. The antimutagenic activity of the methanol extracts was stable at various pH (2, 7 and 10), temperatures (60, 80 and 100℃) and heating times (2, 4, 6, 8 and 10 min at 100℃).