Nonisotopic in situ hybridization has been used to investigate the role of Epstein-Barr virus (EBV) in the aetiology of pediatric Hodgkin's disease. Sections from 24 cases arising in children under the age of 15 years were hybridised with digoxigenin-labelled probes for both EBV and cytomegalovirus, and reactive sites were identified by a sensitive three-layer immunoperoxidase technique. EBV was identified in Reed-Sternberg and mononuclear Hodgkin's cells in five samples (21%). No samples were positive when the cytomegalovirus probe was employed. The specific identification of EBV in the malignant cells of Hodgkin's disease arising in children lends further support for a role of EBV in the aetiology of this disorder.