von Recklinghausen's neurofibromatosis (NF1) is a common inherited human disease. The events leading to patient symptoms from inheritance of a defective NF1 gene are unknown. Since knowledge of the distribution of the normal NF1 gene product should improve understanding of the pathogenesis of the disease, we raised antibodies against peptides coded by portions of the recently cloned human NF1 cDNA. These antibodies specifically recognize a 220 kd protein (neurofibromin) in both human and rat spinal cord. Neurofibromin is most abundant in the nervous system. Immunostaining of tissue sections indicates that neurons, oligodendrocytes, and nonmyelinating Schwann cells contain neurofibromin while astrocytes and myelinating Schwann cells do not. These results suggest a function for neurofibromin in the normal nervous system. Some NF1 disease manifestations, such as Schwann cell tumors and learning disabilities, may result from abnormalities in the cells that express neurofibromin.