Immunocytochemical detection (ICC) of isolated tumor cells in bone marrow (BM) is currently the most established method for monitoring early dissemination in epithelial cancer. However, the low sample size that can practically be analyzed restricts the sensitivity and reliability of the ICC method. To be able to analyze larger samples, a negative immunomagnetic separation (IMS) technique, utilising anti-CD45-conjugated Dynabeads, has been developed. Tumor-cell enrichment by depletion of CD45-expressing mononuclear cells (MNC) is followed by ICC for detection of the cytokeratin (CK)-positive (+) epithelial cells. In this study, bone-marrow samples (n = 165) and peripheral-blood-progenitor-cell (PBPC) apheresis products (n = 22) from breast-cancer patients were analyzed. The negative IMS analysis of 1 to 2 x 10(7) MNC was compared with ICC analysis of 2 x 10(6) unseparated MNC. Negative IMS resulted in 85% mean depletion of MNC. The results showed that 11.7% of the samples were positive by ICC analysis of unseparated MNC, as compared with 23.5% after negative IMS. In samples presenting > 10 CK+ cells, a 4-fold higher number of positive cells was detected by the negative IMS technique. Moreover, there was no evidence for general enrichment of false-positive cells. Altogether our results show that negative IMS is an efficient enrichment method for sensitive detection of CK+ cells in BM/PBPC products from breast-cancer patients. This opens the possibility for further characterization of micrometastatic tumor cells.