During the development of the PNS, Schwann cells (SC) differentiate into myelinating and nonmyelinating cells, implying regulation by different transcription factors such as ZF proteins. Employing an original strategy using monoclonal antibodies specifically directed against the conserved ZF motif, we have identified a new ZF protein of 55 kDa present in rat sciatic nerve extract (SCp55). We used polyclonal antibodies and cloned cDNA to characterize the expression of SCp55 by immunohistochemistry and in situ hybridization. This protein is specific for SC and shows differential expression both during development and between the two SC phenotypes. When they differentiate the protein is first induced in myelinating SC and then in nonmyelinating SC. The nature of this protein together with its differential expression suggests that it is a transcription factor that may have a role in the development of SC.