Autoimmune-prone New Zealand Black mice produce a large amount of autoantibodies cytotoxic for thymocytes (natural thymocytotoxic autoantibodies, NTA). A monoclonal NTA (NTA260) has been found to react with brain tissues as well as the cell surface of thymocytes. We investigated the expression of NTA 260 antigen in the primary culture of fetal brain cells. NTA260 labeled strongly the cytoplasm of nerve cells after fixation, but failed to stain the living cells. Western blot analysis revealed that NTA260 recognized predominantly a band at approximately 53 kDa in brain and thymic extracts. These findings indicate that neuronal NTA260 antigen, which has a molecular mass similar to that of thymocytes, is likely an intracellular component.