In 7 experiments, 1-day- to 4 ½-month-old chickens, free of Marek's disease virus (MDV) and MDV antibody, were exposed to clone-purified virus. Cellular preparations of the GA or JM isolate of MDV were inoculated intra-abdominally, or birds were exposed to G A virus by indirect contact. Genetically susceptible P-line birds were compared to those of the genetically resistant N-line and relatively resistant PDRC strain. Response to infection in various experiments was assessed by: incidence of MD through 56 or 70 days; infectivity of buff y coat, spleen, and kidney cells; presence of viral antigen (fluorescent antibody test) in bursa of Fabricius, spleen, and thymus; and development of precipitin and virus-neutralizing (VN) antibody. Age resistance was characterized by a) lower incidence of MD, b) reduced rate and level of infection 7–10 days post inoculation, and c) development of VN antibody. A strong age resistance developed consistently in the N-line and PDRC birds but was weak or absent in the P-line. Resistance was acquired gradually during several weeks and paralleled the acquisition of competence to make precipitin or VN antibody. During the first 3–6 days after inoculation, infection of lymphoid organs was marked and proceeded equally in all groups, regardless of age or genetic strain. However, by 10–12 days post infection, the level of infection subsided in resistant birds but not in those susceptible.