Bone marrow specimens obtained from 121 breast cancer patients immediately after surgery were examined by an immunofluorescence method with monoclonal antibody MBr1 to detect tumour cells undetectable by other diagnostic procedures. 80 women were node-negative and 41 node-positive. In no case could conventional histology demonstrate tumour cells, whereas MBr1 was positive in 20 (16.5%) of the 121 cases. No difference was observed in MBr1 positivity between node-negative and node-positive cases (17% vs. 15%). With regard to clinical outcome (median follow-up 48 months) 27 women relapsed, including 6 of 20 MBr1-positive and 24 of 101 MBr1-negative patients. First distant metastases or death from progression of disease were taken as end-points. Multivariate analysis showed that the additional contribution of MBr1 positivity, after making allowance for other prognostic factors, was negligible.