A block in carbohydrate chain elongation of O-glycosylated mucins results in accumulation of alpha-GalNAc O linked to serine or threonine (Tn antigen) in a large percentage of human adenocarcinomas. Immunization of mice with desialylated ovine submaxillary mucin (A-OSM), which contains a large concentration of Tn antigen, provided protection against challenge of a highly invasive Tn expressing syngeneic mouse mammary tumor, TA3-Ha. A similar protective effect was not observed in mice immunized with the deglycosylated mucin or irradiated TA3-Ha cells. Immunization with A-OSM but not with deglycosylated mucin resulted in high anti-Tn antibody response in mice. A-OSM induced in vitro proliferation of T-lymphocytes obtained from mice preimmunized with A-OSM or irradiated TA3-Ha cells. This antigen-specific T-cell response was significantly lower if lymphocytes were stimulated with either the deglycosylated or sialylated form of mucin. A-OSM stimulation induced primarily a CD4+ T-cell population, and these cells secreted interleukin 2 in a dose-dependent fashion. A-OSM was also able to induce delayed-type hypersensitivity in mice in response to footpad injections with irradiated TA3-Ha cells. These studies indicate that Tn antigen presented on a protein backbone is capable of providing cellular immunity and protection against tumor in mice.