Peripheral neuronal tumours were studied by the peroxidase-antiperoxidase (PAP) method for the presence of the neurofilament protein (NFP) and neuron-specific enolase (NSE). All cases of ganglioneuromas and ganglioneuroblastomas were positive for NFP and NSE. Both markers were observed only in tumour cells showing differentiation towards ganglion cells. Of the 14 cases of neuroblastoma, 8 were positive for NFP and 12 were positive for NSE. NSE was detected in most neuroblastic tumour cells. However, NFP was found in neuroblasts with signs of differentiation, such as nuclear enlargement, but not in immature, small round cells. NFP was present in cell bodies as well as in cytoplasmic processes of partially differentiated neuroblasts. The majority of pseudorosettes showed no NFP stain. Thus, antibodies against both NFP and NSE are useful in the diagnosis of peripheral neuronal tumours. Moreover, the presence of NFP seemed to be related to the degree of tumour cell differentiation.