Seven-week-old chickens infected oro-nasally with the lentogenic V4 strain of Newcastle disease virus showed no clinical signs and minimal gross pathology. There was slight ulcerative tracheobronchitis but the main system response was a rapid and progressive lymphoproliferative hyperplasia in the respiratory system, gastrointestinal tract, bursa and spleen which tended to peak after three weeks. By using an indirect immunoperoxidase technique with antibody prepared against homologous virus, viral antigen was localised chiefly in the cytoplasm of lymphoreticular cells, persisting for at least 28 days in the caecal tonsil. Positive cells were not seen in the bursa or brain.