A comparative study of immunohistochemical staining for neuron-specific enolase (NSE), protein-gene product 9.5 (PGP 9.5) and S-100 was made in 71 undifferentiated round cell tumours from 65 children using formalin-fixed tissues and a standard alkaline phosphatase-anti-alkaline phosphatase method. All of 29 neuroblastomas marked for NSE and 27 for PGP 9.5; staining was diffuse and usually strong in all tumour elements, irrespective of the degree of differentiation. Patterns of staining remained consistent in primary, recurrent and metastatic tumours and were not modified by previous chemotherapy. S-100 staining was weak and confined to cell processes and schwannian elements in less than half of the tumours studied. Two primitive neuroectodermal tumours both stained strongly for NSE and PGP 9.5. Staining for NSE was observed in single maturing cells in 3/12 rhabdomyosarcomas and in tubular elements in 2/4 Wilms' tumours; primitive rhabdomyoblasts and undifferentiated renal blastema were negative; seven lymphomas were negative. Six of 17 skeletal Ewing's sarcomas showed light to moderate cytoplasmic staining for NSE and PGP 9.5. The site, histology and clinical course of these marker-positive Ewing's sarcomas showed no distinctive features. Staining for PGP 9.5 is a useful additional marker for neural differentiation in round cell tumours.