The binding properties of Brucella abortus smooth lipopolysaccharides (LPS) to the precipitating antibodies in the serum of a B. abortus S2308 infected cow were studied by quantitative precipitin and precipitin-inhibition assays. Crude B. abortus lipopolysaccharides (cLPS) and chemical modification products of strains 1119.3, 19 and 2308 (virulent) were used for this study. In the quantitative precipitin assay, 13 micrograms of cLPS precipitated about 4.0-5.2 micrograms nitrogen of precipitating antibodies in 10 microliter of serum. After alkaline treatment (0.125 N NaOH at 56 degrees C for 1 hr), the alkali-treated lipopolysaccharides (aLPS) from S19 and S1119.3 maintained the maximum precipitability, but the precipitation profile was changed from a sigmoidal to a bell-shaped curve, and aLPS from S2308 was reduced to about 40% of its reactivity. These results indicate that some of the antigenic determinants blocked by other molecules were reshielded by alkaline treatment, and possibly that structural differences exist between virulent (S2308) and nonvirulent (S19 and S1119.3) strains. Among the monosaccharides and glycosides tested for inhibition of precipitation, methyl alpha-D-mannose was most active and about 3.3 times as active as D-mannopyranose (DMan) and only 1/200 as active as acid hapten or the carbohydrate moiety of cLPS prepared from S1119.3 (expressed as a monosaccharide of mol. wt 200) implying that the carbohydrate determinants of this antigen are probably related to a DMan alpha 1----linked compound. The precipitability of the antibodies in this serum was greatly reduced after absorption with cLPS and the precipitin reaction was completely inhibited by the carbohydrate moiety of LPS (O-antigen) S1119.3 demonstrating that the common antigenic determinants (hapten) of B. abortus LPS reacting with these precipitating antibodies are in the region of the carbohydrate moiety of the LPS.