The detection of early micrometastasis or disseminated single tumor cells poses a problem for conventional diagnosis procedures. Using a panel of monoclonal antibodies against cytokeratin and the 17-1A epithelial antigen we identified immunocytochemically tumor cells in bone marrow of patients with breast cancer (n = 155) and colorectal cancer (n = 57) at the time of surgery of the primary tumor. Monoclonal antibody CK2, recognizing the human cytokeratin component 18 in simple epithelia, appeared to be the most suitable reagent because of its negative reaction with bone marrow samples of the noncarcinoma patients (n = 75). Its specificity was further demonstrated in a double-marker staining procedure using an anti-leukocyte common antigen monoclonal antibody (T200) as counterstain. A comparative analysis showed that immunocytology was clearly superior to conventional cytology (n = 212) and histology (n = 39). In 9.5-20.5% of patients without distant metastasis, tumor cells could be detected in bone marrow. We found a significant correlation between tumor cells in bone marrow and conventional risk factors, such as distant metastasis or lymph node involvement. In a first approach toward immunotherapy we demonstrated in 3 patients that infused monoclonal antibody 17-1A can label single tumor cells in bone marrow in vivo. We then used this approach to follow up 7 patients undergoing 17-1A therapy in an adjuvant clinical trial.