Immunohistochemical (IHC) techniques should allow for a greater detection of bone marrow micrometastasis in patients with breast carcinoma. We studied a series of bone marrow (BM) biopsies negative by conventional histologic techniques from 93 patients with breast carcinoma. Prior to this study, twelve BM biopsies, positive by conventional histology, were stained with a panel of monoclonal antibodies (MoAb), directed either against cytokeratin (KL1, AE1-AE3, CAM5-2) or epithelial membrane antigen (EMA, HMFG2). KL1 appeared to be the most sensitive of the markers used in the detection of metastases and is available commercially. It therefore was the only MoAb used with the series of 93 BM biopsies negative by conventional examination. Within this series, among 45 patients clinically suspected of having bone marrow metastasis but with BM biopsies negative by conventional staining, one case showing myelofibrosis stained positive with KL1 demonstrating isolated tumor cells. For the 48 patients without suspicion of bone marrow metastasis at initial diagnosis for breast carcinoma, KL1 revealed no marrow metastasis. Single bone marrow biopsy techniques whether stained by conventional or IHC methods do not appear to be useful tests to detect occult bone marrow metastasis, especially at initial diagnosis of clinically Mo breast carcinoma patients.