The immunotherapy of cancer requires the definition of a suitable target for and the induction of a CD8+ cytotoxic lymphocyte reaction. In breast cancer, particularly mucins (MUC1) of the variable number of tandem repeat sequence may be a suitable target, but there has been a problem in inducing a cytotoxic response. MUC1 peptides, conjugated to carriers (keyhole-limpet hemocyanin or diphtheria toxoid) induce a humoral response and give poor tumor protection in mice and there is little cellular immunity. However, when MUC1 fusion protein is conjugated to mannan under oxidizing conditions, a cellular immune response is induced, with significant tumor protection, cytotoxic T lymphocytes and little antibody. The procedure may also be useful for other antigens.