Flow cytometry has been compared with conventional cytology as a method for detecting breast carcinoma cells in human bone marrow. Breast cancer cells from patient effusion fluids or from established cell lines were mixed with normal human bone marrow at dilutions ranging from 1:10 to 1:100,000. Cells were labeled with five directly fluoresceinated murine monoclonal antibodies reactive against epithelial cell surface determinants of 42, 55, 72, 200, and more than 200 kD. Fluorescence of tumor cells within the mixtures was then analyzed with the use of a fluorescence-activated cell sorter. In multiple experiments, one tumor cell in 10,000 nucleated marrow cells could be detected by flow cytometry. In addition, a linear correlation was observed over approximately 3 logs between the number of tumor cells added and the number of tumor cells detected. In a double-blind study comparing flow cytometry and cytology, flow cytometric analysis detected one tumor cell among 10,000 marrow precursors in 14 of 15 instances, whereas standard cytologic methods detected similar tumor contamination in 9 of 15 instances. Neither technique used individually could detect one tumor cell in 100,000 bone marrow cells. Used in combination, however, flow cytometry and cytology could detect one breast cancer cell among 100,000 normal marrow progenitors.