A sequential study of cytotoxic and mitogen stimulation responses of peripheral blood leukocytes (PBL) after inoculation with Marek's disease (MD) virus was done with the 7 x 15 chickens at 2 weeks of age (group 1) and at 12 weeks (group 3). Control groups were inoculated with chicken kidney cells at 2 weeks of age (group 2) and 12 weeks (group 4). During an 8-week observation period, morbidity and mortality were greater in group 1 chickens than in group 3 chickens. The cytotoxic response of PBL against an MD tumor cell line (MSB-1) was greater during the first 2 weeks after chickens in group 3 were inoculated than in group 1 chickens. Correlations did not exist between cell-mediated cytotoxic responses and severity of disease for individual group 1 chickens. However, PBL fo group 3 chickens that developed gross lesions of MD had reduced cytotoxic activities after an initial activity equal to, or greater than, the activities of chickens that survived without gross lesions. Phytohemagglutinin stimulation of PBL was depressed in both groups during weeks 1 to 3 after inoculation. Group 3 chickens had higher virus titers in PBL than group 1 chickens during weeks 1 to 3 after inoculation. The titers subsequently decreased below those for group 1 during the remainder of the experiment.