We have compared three different RT-PCR procedures to measure cytokeratin 19 (CK19), carcinoembryonic antigen (CEA) and mucin MUC1 gene expression in order to determine their diagnostic value in detecting tumour cells in bone marrow aspirates of patients with operable breast cancer. In an experimental model, the best sensitivity was observed for CK19 and MUC1 RT-PCR assays, although only the CEA and CK19 assays showed good specificity. The study of 42 patients showed that a 'CK19 positive/CEA positive' RT-PCR assay in bone marrow correlated positively with a positive axillary lymph node status (N(0) versus N(1-3), P<0.05). Both assays were also positive in 17% of node negative patients. RT-PCR assays were more sensitive in bone marrow than in peripheral blood. Our results suggest that CK19 and CEA RT-PCR assays are powerful methods for detecting disseminated breast cancer cells. A larger study with long-term follow-up is required in order to clarify their clinical usefulness.