Immunomagnetic selection of epithelial cells from peripheral blood was combined with flowcytometric or microscopic analysis to characterize and enumerate epithelium derived tumor cells in the circulation of patients with carcinoma of the breast. Highly significant differences in the number of circulating epithelial cells were found between normal controls and patients with breast cancer. In addition, significant differences in the number of circulating epithelial cells were found in patients in whom the tumor is confined to the primary tumor and those with metastatic disease to local lymphnodes or distant sites . The malignant nature of the cells was demonstrated by their cytology and immunophenotype. In following up the number of epithelial cells in the blood of patients with metastatic carcinoma of the breast, it was shown that it was well correlated with the activity of the disease.