The acetohydroxyacid synthase (ARAS) of Brucella suis can be effectively targeted by the sulfonylureas chlorimuron ethyl and metsulfuron methyl. Growth in minimal medium was inhibited, and multiplication in human macrophages was totally abolished with 100 mu M of sulfonylureas. Metsulfuron methyl-resistant mutants showed reduced viability in macrophages and reduced AHAS activity.