Interaction between antigen presenting cells and T lymphocytes, through receptors and co-stimulatory molecules present on the surface of these cells, is one of the key means to modulate the adaptive immune system. Tucaresol, a Schiff-base-forming chemical, can be used as a substitute for the physiological donor of carbonyl groups of antigen presenting cells, which can interact with the amine groups of T lymphocytes to modulate the adaptive immune system. This study was done to determine whether tucaresol can enhance killing of cancer cells in vitro as well as protect non-obese diabetic severe combined immunodeficient mice from tumor development by mucin 1 stimulated human mononuclear cells through the adaptive immune system. The expected hypothesis was not supported. Percent specific lysis of MCF-7 tumor cells by mucin 1 stimulated human mononuclear cells was reduced by tucaresol. Furthermore, tucaresol abolished the protective effect of mucin 1 stimulated human mononuclear cells against MCF-7 breast cancer cell growth in non-obese diabetic severe combined immunodeficient mice. This study implies that tucaresol may be of use as an immunosuppressive agent.