A highly sensitive assay is disclosed which combines immunomagnetic enrichment with multiparameter flow cytometric and immunocytochemical analysis to detect, enumerate and characterize carcinoma cells in the blood. The assay can detect one epithelial cell or less in 1 ml of blood and has a greater sensitivity than conventional PCR or immunohistochemistry by 1-2 orders of magnitude. In addition, the assay facilitates the biological characterization and staging of carcinoma cells.