We report a highly sensitive method to detect rare human breast cancer cells, which combines an immunomagnetic separation (IMS) using antibody BM2 against MUC-1 with cytokeratin-19 (CK19) and the reverse transcriptase/polymerase chain reaction (RT-PCR). The IMS-RT-PCR technique allows the detection of 1 tumor cell/10(7)-10(8) mononuclear cells. This is at least ten times more sensitive than CK19 RT-PCR alone, or immunocytochemistry. All 117 peripheral blood and 8 bone marrow samples obtained from healthy donors as negative controls were positive for beta2-microglobulin by RT-PCR but negative for CK19 by IMS-RT-PCR or RT-PCR alone. Out of 26 bone marrow samples from breast cancer patients, 18 had CK19 transcripts detectable by IMS-RT-PCR. In contrast, only 14 and 13 samples from the 26 were found to be positive by RT-PCR alone or by routine immunocytochemical staining. In conclusion, IMS-RT-PCR for CK19 is a highly sensitive and specific method for detecting very low numbers of micrometastatic breast cancer cells in bone marrow amidst an excess of nonmalignant cells. For the early diagnosis of disseminating disease, this assay is more efficient than RT-PCR alone and routine immunocytochemistry.