A significant percentage of women with primary breast cancer have micrometastatic disease in the bone marrow. Bone marrow involvement may be an adverse prognostic factor, and more aggressive therapy may be indicated for these patients. There are a number of different techniques and antibodies used to detect tumor cells in the bone marrow. We used the same panels of four antibreast cancer antibodies and compared three immunodetection techniques: two-color immunofluorescence, immunohistochemical staining, and fluorescence-activated cell sorting with cytologic examination of the sorted cells. The two-color immunofluorescence technique was superior and consistently detected one tumor cell contaminating one million normal bone marrow cells and had no reactivity with normal bone marrow.