Multivalent immunogenic molecules comprise a carrier molecule containing at least one functional T-cell epitope and multiple different carbohydrate fragments each linker to the carrier molecule and each containing at least one functional B-cell epitope. The carrier molecule inputs enhanced immunogenicity to the multiple carbohydrate fragments. The carbohydrate fragments may be capsular oligosaccharide fragments from Streptococcus pneumoniae, which may be serotypes 1, 4, 5, 6B, 9V, 14, 18C, 19F or 23F, or Neisseria meningitidis, which may be serotype A, B, C, W-135 or Y. Such oligosaccharide fragments may be sized from 2 to 5 kDa. Alternatively, the carbohydrate fragments may be fragments of carbohydrate-based tumor antigens, such as Globo H, Le^Y or STn. The multivalent molecules may be produced by random conjugation or site-directed conjugation of the carbohydrate fragments to the carrier molecule. The multivalent molecules may be employed in vaccines or in the generation of antibodies for diagnostic application.