Synthetic overlapping oligosaccharide fragments of Streptococcus pneumoniae serotype 14 capsular polysaccharide (Pn14PS), {6)-[beta-D-Galp-(1 -> 4)-] beta-D-GlcpNAc-(1 -> 3)-beta-D-Galp-(1 -> 4)-beta-D-Glcp-(1 -> 3}(n), were conjugated to CRM197 protein and injected into mice to determine the smallest immunogenic structure. The resulting antibodies were then tested for Pn14PS specificity and for their capacity to promote the phagocytosis of S. pneumoniae type 14 bacteria. Earlier studies have reported that the oligosaccharide corresponding to one structural repeating unit of Pn14PS, i.e., Gal-Glc-(Gal-) GlcNAc, induces a specific antibody response to Pn14PS. The broader study described here, which evaluated 16 oligosaccharides, showed that the branched trisaccharide element Glc-(Gal-) GlcNAc is essential in inducing Pn14PS-specific antibodies and that the neighboring galactose unit at the nonreducing end contributes clearly to the immunogenicity of the epitope. Only the oligosaccharide conjugates that produce antibodies recognizing Pn14PS were capable of promoting the phagocytosis of S. pneumoniae type 14. In conclusion, the branched tetrasaccharide Gal-Glc-(Gal-) GlcNAc may be a serious candidate for a synthetic oligosaccharide conjugate vaccine against infections caused by S. pneumoniae type 14.