A vaccine comprising purified outer-polysaccharide (OPS) is effective for protection against brucellosis. The vaccine is derived from Brucella or a variety of cross reactive bacteria. The vaccine can be administered by different routes (intramuscularly, subcutaneously, intraperitoneally, orally). The vaccine is effective in protecting against other infectious bacteria, aside from Brucella. It is likely that the vaccine can be given after infection to reduce illness.