The invention pertains to conjugates of the capsular polysaccharide of Vibrio cholerae O139, or a structurally and/or immunologically related oligo- or poly-saccharide, and a carrier. These conjugates are useful as pharmaceutical compositions and/or vaccines to induce serum antibodies which have bactericidal (vibriocidal) activity against V. cholerae, in particular V. cholerae O139, and are useful to prevent, treat and/or reduce the severity of disease caused by V. cholerae infection, such as cholera. The present invention also relates to diagnostic tests for V. cholerae infection, and/or cholera caused by V. cholerae infection, using one or more of the oligo- or poly-saccharide-carrier conjugates or antibodies described above.