Polymorphic epithelial mucin (MUC1) was detected in myeloma cells and in sera of multiple myeloma patients. HLA-unrestricted CTL that recognize tumor-associated epitopes on MUC1 has been shown to be induced from breast and pancreas cancer patients. To investigate whether such CTL can also be induced from multiple myeloma patients, an allogeneic mixed leukocyte tumor cell culture was performed. PBMCs of a multiple myeloma patient were stimulated by different allogeneic breast carcinoma and myeloma cell lines. The cultured PBMCs were proliferated and a CTL line TN was established. TN exclusively expressed TCR-alpha/beta, CD3, and CD8. TN lysed breast carcinoma and myeloma cell lines but did not lyse K562, which is sensitive to NK cells. The cytotoxicity of TN was inhibited by anti-CD3 Abs but not by anti-HLA Abs. Thus, the TCR-alpha/beta was considered to be involved in the recognition of the target cells but HLA was not. Furthermore, TN lysed transformed mouse fibroblast cells transfected with MUC1 cDNA, suggesting that this CTL line recognizes MUC1 directly. Thus, it is concluded that precursors of HLA-unrestricted and anti-MUC1 reactive CTL could exist in the peripheral blood of multiple myeloma patients and that myeloma cells can express epitopes on MUC1, which can be recognized by the CTL.