The common acute lymphoblastic leukemia antigen (CALLA), CD10, is a 100-kDa surface glycoprotein endowed with neutral endopeptidase activity, shared by a number of hemopoietic and non-hemopoietic cells. In this report, immunohistochemical and Western blot techniques, using different anti-CD10 monoclonal antibodies, were utilized to demonstrate that CD10 is also expressed by myelin sheaths of the human peripheral nervous system (PNS), but not of the central nervous system. CD10-positive immunoreactivity appeared to be localized in the outer and inner borders of myelinated fibers, in nodes of Ranvier and in the Schmidt-Lantermann clefts, thus showing a distribution pattern very similar to that of myelin-associated glycoprotein (MAG). The above findings suggest that CD10 antigen, through its enzymatic activity, may have a functional role in the assembly and maintenance of PNS myelin. In addition, it is not known whether CD10, similarly to MAG, may be a target antigen in some PNS immune-mediated disorders.