MUC1 is a highly immunogenic epithelial mucin and serves as a tumor-associated antigen in breast, pancreatic and ovarian carcinomas. We previously reported the expression of MUC1 on myeloma cells and the establishment of an HLA-unrestricted cytotoxic T lymphocyte (CTL) line TN that recognized MUC1 from peripheral blood mononuclear cells in a multiple myeloma patient. In this study, we attempted to induce such CTL from six other multiple myeloma patients consecutively in order to show that the induction of the CTL line TN had not resulted from some idiosyncrasy of the first patient. Bone marrow mononuclear cells were used to induce CTL, because they contain myeloma cells that might stimulate the autologous lymphocytes. Bulk CTL lines were induced from two out of six patients. The CTL line TS was CD8+ cell dominant and KY was CD4+ cell dominant. Both CTL lines lysed MUC1+ myeloma and breast carcinoma cell lines. The cytotoxicity of the CTL lines was inhibited by anti-CD3, anti-alpha beta TCR and anti-MUC1 mAb. It was also inhibited by a MUC1 transfectant, but not by a mock transfectant in cold target inhibition assays. MUC1 was transfected into a human colonic carcinoma cell line. The reactivity of anti-MUC1 core protein mAb and the cytotoxicity of the CTL against the transfectant was enhanced by the treatment of the cells with an O-glycosylation inhibitor. Thus it is generally accepted that the HLA-unrestricted CTL which directly recognize the underglycosylated from of MUC1 using their TCR could be induced from a certain proportion (approximately 30%) of untreated multiple myeloma patients.