Some human tumors express known antigens that can be utilized as targets for specific immunotherapy. An absolute requirement for the efficacy of this therapeutic strategy is an adequate expression of the candidate antigen by all cells of the primary and metastatic tumor. To examine the presence and distribution of tumor‐associated antigens in metastatic breast cancer, we used PCR analysis and ethidium bromide staining to test the expression of genes of the MAGE family in 28 primary tumors and related metastatic samples. Overall, samples obtained from 7 of 28 patients revealed positive. However, 2 of 3 primary tumors positive for MAGE‐1 and/or MAGE‐3 had corresponding negative metastatic lesions. On the contrary, 4 of the 25 MAGE‐negative primary tumors gave rise to positive metastatic nodes. Our results confirm in vivo, at the molecular level, the tumor‐antigen heterogeneity previously observed at the cellular level by in vitro analysis. Our data strongly suggest that, at least in patients with breast cancer, multiple different antigens would be required to optimize the recognition of neoplastic cells in immunotherapeutic protocols using MAGE products as target antigens. © 1995 Wiley‐Liss, Inc.