This paper describes the immunohistochemical staining properties of four monoclonal antibodies (MAbs) (CF, EB, AD, and KB) which had been previously shown to be specific for purified neuron-specific enolase (NSE) by a solid-phase radioimmunoassay. In this study, the authors immunostained a spectrum of normal and neoplastic neuronal, "neuroendocrine," and nonneuronal tissues fixed in formalin and embedded in paraffin. Positivity was generally restricted to normal neuronal structures and neuronal tumors, including adrenal neuroblastoma, ganglioneuroblastoma, olfactory neuroblastoma, pheochromocytoma, carotid body paraganglioma, duodenal gangliocytic paraganglioma, and teratoma with neuroepithelial components. Three staining patterns of the normal or neoplastic neuronal structures were observed: two MAbs (CF and EB) stained predominantly the nerve fibers (axoplasm); one (AD) stained predominantly the cell bodies (perikaryon); and one (KB) stained both the axoplasm and the perikaryon. "Neuroendocrine" tumors such as pulmonary small cell carcinoma, pancreatic islet cell tumor, thyroid medullary carcinoma, and carcinoid tumors from various locations showed a variable staining pattern. Tumor cells undergoing mitotic division were usually positive regardless of type. Normal structures other than neuronal or "neuroendocrine," including normal glial cells, were negative. The authors also studied a range of glial cell tumors with MAbs CF and AD as well as with Dako polyclonal antiserum to NSE. The results showed that CF stained the axonal fibers in the normal white matter surrounding these tumors; it did not stain the tumor cells or the perikarya of neurons in the surrounding normal gray matter. AD stained the glioma cells as well as the perikarya and dendrites of neurons in the surrounding normal gray matter; it did not stain the axonal fibers in the surrounding normal white matter. By contrast, the polyclonal antiserum stained all of these structures. The high degree of staining specificity of the MAbs should prove them to be valuable in immunohistochemical diagnosis of tumors as well as in further understanding the role of NSE in neuronal differentiation.