As part of our program on Tn-specific anti-tumor immunotherapy, our aim was to vary the nature of the aglyconic part of the tumor-associated Tn antigen (alpha-d-GalNAc-Ser/Thr). This report describes the synthesis of Fmoc-hSer-(alpha-d-GalNAc)-OH (4) in 19% overall yield from protected aspartic acid. The building block 4 was incorporated as trimeric clusters into a glycopeptide vaccine [MAG:Tn(hSer)3-PV], using solid-phase peptide synthesis. When injected in mice, the resulting MAG induces a strong antibody response, which recognizes native tumor-associated antigens (TAA) at the surface of human tumor cells. This approach may be extended to the use of other nonnatural TAA in order to improve half-life of synthetic anti-cancer vaccines.