Sixty human nervous system neoplasms were examined by immunohistochemistry using a monoclonal antibody against neurofilament triplet proteins. Only those of neuronal origin had tumor cells with intracytoplasmic, immunoreactive neurofilament triplet proteins. However, not all such neoplasms contained labeled tumor cells. Benign or differentiated neuronal tumors more often contained labeled cells than malignant, less differentiated neoplasms of the neuron series. We conclude that anti-neurofilament monoclonal antibodies are useful reagents for the evaluation of human neoplasms.