Organ cultures of lymphomas from chickens infected with Marek's disease (MD) virus were examined by electron microscopy at various periods after explantation. Non-enveloped herpes-type virions were observed in explants 18 days after the culture was initiated. Cytolytic virus was replicated often in lymphoblastoid cells and occasionally in other cell types (permissive and nonpermissive transformed). The permissive cells were numerous, but nonpermissive transformed cells were also present as indicated by the longevity of some cultures. The presence of both permissive and transformed cell types in the organ cultures and reasons for their sequential appearance were discussed. The results unequivocally demonstrated that lymphoid cells of tumors induced by MD virus were sites of virus replication under proper culture conditions.