Trichloroacetic (TCA) extraction of B. melitensis 115, or hot saline extraction of B. melitensis 16M yields a polysaccharide component that can be purified free of protein and lipopolysaccharide by gel filtration and TCA precipitation. Antibodies to this polysaccharide can be detected in sera of rabbits infected with virulent Brucella but not with B. abortus 19. Similarly, cattle with active brucellosis and humans with acute brucellosis develop antibodies against this polysaccharide. After extraction and purification, this polysaccharide does not elicit antibody response in rabbits or mice. It does not fix the complement, does not bind to red cells or to polystyrene plates.