Patterns of demyelination were studied in sciatic nerves, spinal roots and ganglia of chickens afflicted with either Marek's disease (MD) or experimental allergic neuritis (EAN). MD was induced in susceptible chicks after hatching by inoculation of the JM strain of MD Herpes virus. Tissues from these chickens were examined 7-83 days after infection. EAN was studied 10-21 days after sensitization of 4 week old chickens to emulsions containing human peripheral nerve with complete Freund's adjuvant. In both conditions lesions were encountered which consisted of perivenular infiltrates of mononuclear cells that penetrated the basal lamina of the neurolemmal sheath, displaced Schwann cells, lysed and stripped myelin lamellae without damage to axons. Other lesions in MD were characterized by lymphomatous infiltrates that contained necrotic cells and disintegrating axons. The similarity of the demyelinating process in MD to that seen in EAN suggests that MD virus infection activates lymphocytes sensitized to peripheral nerve myelin. The findings are discussed with reference to acute idiopathic polyneuritis (Guillain-Barré syndrome) in patients with preceding or concurrent Herpes virus infections including those known to cause lymphoproliferative disorders.