A new monoclonal antibody NB84 recognizing an uncharacterized molecule of 57 kD was produced using human neuroblastoma tumour tissue as a source of antigen. The antibody was selected for its ability to give reliable staining on conventional formalin-fixed, paraffin-embedded material. In the present study, NB84 was assessed for its usefulness in the differential diagnosis of paediatric small cell tumours. It gave reliable staining of 17/19 (90 per cent) neuroblastomas and was negative on all other tumours apart from Ewing's sarcomas, of which 9/14 (64 per cent) showed mainly focal positivity. It is concluded that in association with a panel of markers against leucocyte, muscle, epithelial, and neural markers NB84 provides considerable assistance in the recognition of neuroblastoma and thus is important in enabling the most appropriate therapy to be given.