Sialyl Thomsen-nouveau (STn) is a tumor-associated carbohydrate antigen (TACA) that is overexpressed hi a variety of carcinomas such as breast, ovarian, and colon cancer. In normal tissue, STn is not detectable, which is critical for opportunities in developing cancer immunotherapies. A novel, entirely carbohydrate, semisynthetic STn-polysaccharide (PS) A1 conjugate was prepared and evaluated in CS7BL/6 mice. STn-PS A1 was combined with commercially available monophosphoryl lipid A-based adjuvant, and after immunization, ELISA indicated a strong immune response for inducing anti STn IgM/IgG antibodies. The specificity of these antibodies was concomitantly investigated using FACS analysis, and the results indicated excellent cell surface binding events to STn-expressing cancer cell lines MCF-7 and OVCAR-5. An INF-gamma ELISpot assay was conducted to further confirm a robust cellular immunity invoked by STn-PS A1. Most importantly, the raised antibodies conferred complement-dependent cellular cytotoxicity against MCF-7 and OVCAR-5 cells.