This paper reports the chemical and electrophoretic properties of different fractions of Brucella abortus and Brucella melitensis lipopolysaccharides (LPS), and polysaccharide B (Poly B) from Brucella melitensis. Also the mitogenic effect of these fractions on murine spleen cells is studied. LPS fractions (F-5 and F-3) were obtained by water-phenol extraction and the poly B from autoclaved bacteria. For each bacterial fraction the KDO, protein and nucleic acid content, the electrophoretic mobility in polyacrylamide gel and its capacity to induce blast transformation in vitro, or the ability to produce an increase of splenic weight when injected intraperitoneally in mice was determined. Protein and KDO content in F-5 from B. abortus and B. melitensis were similar; however F-3 from B. abortus showed less KDO but more protein and nucleic acid than F-3 from B. melitensis. Poly B showed higher protein content as compared with the other two fractions. The electrophoretic pattern of F-5 in both Brucella species was similar, but different from F-3 and Poly B. In the blast transformation assay it was found that F-5 is the most active fraction in both Brucella species. It is concluded that the high lymphoblastic effect induced by Brucella polysaccharide is mainly due to lipid A, but the repeating units of sugar in the ''O'' chain or in the Poly B also participate.