Two hundred and fifty bone marrow and 140 lymph nodal biopsies were analyzed immunocytochemically, using a mouse monoclonal antibody b-12 (MAb b-12), which reacts with MCA (mucinous-like carcinoma-associated antigen). The presence of MCA in bone marrow specimens was demonstrated in 102 out of 105 (97.1%) breast cancer metastases, 5 out of 8 (62.5%) gastric cancers, 5 out of 6 (83.3%) colon cancers, 3 out of 5 (60%) prostate cancers, 11 out of 26 (42.3%) lung cancers and 25 out of 30 (83.3%) unknown primary cancers, while no positivity to anti-MCA antibody was found in 30 cases of normal bone marrow biopsies, 5 cases of non epithelial malignancies and 30 cases of hemolymphoproliferative disease. Normal lymph nodes and non-epithelial lymph node metastases did not show any reaction to MAb b-12; on the contrary MCA positive staining was observed in 75 out of 75 (100%) lymph nodal metastases in breast cancer. These results suggest that application of MAb b-12 in immunohistochemistry is valid for the detection of bone marrow and lymph nodal micrometastases of epithelial origin.