The MAGE gene family, encoding tumor-rejection antigens recognized by cytotoxic T lymphocytes, is frequently expressed in human solid cancers. However, its expression in leukemia has not been well studied. We have investigated MAGE gene expression at the mRNA level in human leukemia. The MAGE gene family was expressed in 17 of 34 (50%) examples of T cell leukemia (12/21 patients' peripheral blood mononuclear cells and 5/13 cell lines), in 7 of 16 (44%) cases of B cell leukemia (1/8 and 6/8 respectively), but in none of 23 myelomonocytic leukemia cases (0/16 and 0/7), as evaluated by the primers common to the MAGE-1, -3, -4 (-4a and/or -4b), and -6 genes and the semi-quantificative reverse transcription/polymerase chain reaction method. None of a panel of normal lymphoid cells expressed the MAGE gene family. As revealed by the primers specific for each of the MAGE genes, the MAGE-1, -2, -3, -4 or -6 gene was expressed in 8, 8, 6, 2, or 6 respectively out of 23 types of leukemia cell lines. Expression of the MAGE-1 protein in both the cell lines and patients' cells was confirmed by immunoblot analysis with the polyclonal antibody to recombinant MAGE-1 protein. Cellular MAGE-4 protein in the cell lines was measured by an enzyme-linked immunosorbent assay with the polyclonal and monoclonal antibodies to recombinant MAGE-4b protein. In summary, the MAGE gene family was found to be expressed in the substantial proportion of T cell leukemias, but in no case of myelomonocytic leukemia. Antigens coded by the MAGE gene family could be important molecules for understanding specific immunity against lymphocytic leukemia.