A bacterial reverse-mutation assay with Salmonella Typhimurium TA1535, TA1537, TA98, TA100, and TA102 and an in vitro chromosome aberration assay with Chinese hamster lung (CHL) cells were used to investigate the genotoxicity of the methanol extract of Korean soybean paste (doen-jang [dwen-jahng]) and its antigenotoxic activity against aflatoxin B1. The methanol extract revealed nonmutagenic potential for all of the bacterial strains tested. The extract significantly reduced the numbers of revertants per plate when it was added to the assay system with Salmonella Typhimurium TA100 (P < 0.05). The extract also exhibited significant inhibitory effects on chromosome aberration in CHL cells (P < 0.05). The findings of this work indicate that the methanol extract of Korean soybean paste could have strong potential as an antigenotoxic material.