Among the human mucins, MUC1 is unique in its cell-surface transmembrane expression and its apparent signal-transduction functions. The high expression of MUC1 on many human cancers makes it an attractive target for immunotherapy. Immunization of human cancer patients with MUC1 peptides has resulted in the generation of both anti-MUC1 antibody and cytotoxic T lymphocyte responses. Recently, a novel immunoregulatory role for MUC1 has been suggested by experiments demonstrating that soluble MUC1 induces T-cell unresponsiveness, and that T cells appear to express and secrete MUC1 following their activation. MUC1 is an apparent paradox, having both adhesive and antiadhesive functions, and immunostimulatory and immunosuppressive activities.