We immunostained histologic sections of formalin-paraffin-processed specimens from 112 neuroendocrine tumors to evaluate the expression of intermediate filaments, especially neurofilament (NF) protein. The commercially available anti-human NF protein monoclonal antibody was raised against the Mr 200,000 and 70,000 components of neurofilaments. None of our tumors exhibited unequivocal immunoreactivity for Mr 200,000 or 70,000 neurofilament proteins. The failure of a tumor to express the Mr 200,000 or 70,000 component of NF protein does not question its neuroendocrine nature. However, the unequivocal positivity for Mr 200,000 or 70,000 components of NF in an undifferentiated tumor with a differential diagnosis between neuroendocrine small-cell carcinoma and a neurogenic tumor should support the latter consideration.