Expression of aminopeptidase N (CD13) in mesenchymal tumors. For a long time, CD13 molecules have been considered to be restricted to myeloid cells and related neoplasms. Meanwhile, however, expression of CD13 has also been detected in some hepatocellular, gallbladder, renal, and lung carcinomas, and even in some fibrosarcomas and malignant melanomas. In this study, expression of CD13 antigen was immunohistochemically examined in non-neoplastic mesenchymal cells, along with 33 benign and 83 malignant mesenchymal tumors (MET) using CD13 monoclonal antibodies (MAb) My7, U71, WM-15, and MoU48. In non-neoplastic mesenchymal cells, expression of CD13 was restricted to perivascular fibrocytes/blasts, tissue histiocytes, osteoclasts, and to the perineurium of peripheral nerve trunks. Under neoplastic conditions, CD13 was detectable in some tumors of smooth muscle, fibrous, fibrohistiocytic, synovial, osteogenic, and peripheral nerve sheath origin, and even in some tumors of adipose tissue. Tumors of striated muscle origin, of autonomic ganglia, and of cartilage-forming tissues were CD13-negative throughout. Thus in most but not all tumors studied the pattern of expression of CD13 mirrors the situation found in their cells of origin. These findings enrich the data on expression of leukocyte differentiation antigens in extra-hematopoietic tissues. Expression of CD13, which meanwhile is known to be identical to aminopeptidase N, an important peptide-cleaving enzyme, in only some MET might reflect a special functional state of these neoplasms. 