The mammaglobin gene encodes a novel, breast cancer-associated glycoprotein. In this study, we have evaluated the frequency with which mammaglobin expression can be detected in primary and metastatic breast tumors and in breast tumor cells present in the peripheral circulation. Of 100 primary human breast tumors examined, 81 were strongly immunopositive for mammaglobin protein. Staining was independent of tumor grade and histological type. Ten of 11 lymph nodes from patients with metastatic breast cancer contained detectable mammaglobin mRNA, whereas mammaglobin expression in uninvolved lymph nodes was undetectable. Using a nested reverse transcription-PCR assay, mammaglobin mRNA was also detected in 9 of 15 products (60%) used for autologous stem cell transplant. These results suggest that larger clinical studies are warranted to investigate the full clinical utility of mammaglobin as a tool for breast cancer patient management.