The 13C nuclear magnetic resonance spectrum of Brucella melitensis native polysaccharide hapten proved to be very similar to that generated by the O-specific chain (PS) isolated from B. melitensis lipopolysaccharide; that is, to a linear polymer in which the repeating unit is composed of five N-formylperosaminyl residues, one of them being substituted at position C-3 and the other four at position C-2. The serological analysis suggests that the so-called A determinant is present solely in Brucella abortus PS, the M determinant is only in B. melitensis PS, and the extensive cross-reaction observed is due to a determinant shared by both polysaccharides.