It is possible to detect micrometastases in primary breast cancer using immunocytochemical staining of bone marrow smears. However, using the light microscope the procedure is time-consuming and laborious because such cells occur rarely (less than 1 in 10,000). Using an image analysis system, the Leytas machine, and a specially prepared reproducible slide it has been possible to automate the technique. A 100% concordance was found between the machine and the light microscope in the identification of slides containing moderate to high numbers of tumour cells in bone marrow, and in those containing no tumour cells. However, in those slides containing low numbers of tumour cells (1-10 tumour cells/10(6) normal bone marrow cells) the sensitivity was decreased to 91%. In the presence of non-specific staining the false positive rate was increased from 0% to 22%. This method represents a potential improvement in the assessment of an important clinical staging procedure.