Summary Electron microscopy of short-term cultured blood lymphocytes from spontaneous cases of “classical” (neural) Marek's disease and some clinically normal chickens from the same flock showed a herpes-type intranuclear virus in a high proportion of transformed cells. In a few instances virus particles were closely associated with intranuclear filaments, which were thought to represent the product of aberrant viral replication. Some non-transformed cells showed granule-lamellar arrays in the cytoplasm, thought possibly to represent the inner structure of the filaments. The addition of cultured lymphocytes to healthy chick kidney monolayers induced a typical transmissible cytopathic effect associated with nuclear inclusions. Similar results were obtained using fresh lymphocytes, or tumour cells, from cases of the “acute” disease. A spontaneous cytopathic effect often developed in monolayers prepared from clinically normal chickens in a flock in which the disease was endemic. Indirect immunofluorescence tests with either classical or acute Marek's disease antisera indicated the presence of viral antigen in a high proportion of transformed lymphocytes and in the kidney cells in the region of the cytopathic effect. A herpes-type virus was also demonstrated in cultured transformed blood lymphocytes in cases of acute Marek's disease with gonadal tumours.