A novel reverse transcription-polymerase chain reaction (RT-PCR) assay using mammaglobin B gene was developed for detection of breast cancer micrometastases in axillary lymph nodes. Fourteen primary breast cancers and 56 axillary lymph nodes from six patients with primary breast cancer and 15 control lymph nodes from non-cancer bearing patients were subjected to this assay. The transcript of mammaglobin B gene was detected in none of the control lymph nodes, but in all of the 14 primary breast cancers. Eleven out of the 56 lymph nodes from the patients, which were shown to be positive by histological examination, were also proven positive by this assay. On the other hand, fourteen of the 45 (31%) histologically negative lymph nodes were also shown to express mammaglobin B mRNA, which suggested the presence of micrometastases in these lymph nodes. RT-PCR using mammaglobin B gene could therefore be a useful tool for detection of micrometastases of breast cancer.