The cell-mediated immune responses of the chickens inoculated with the cell-associated (CA) ILT vaccine were studied. Lymphocyte blastogenic response was tested by MTT assay with spleen, thymus, bursa of Fabricius and peripheral blood lymphocytes of the vaccinated chickens. The blastogenic response of peripheral blood and spleen lymphocytes was enhanced by a T-cell mitogen. The CA vaccine induced an immunity satisfactorily in bursectomized chickens. To study the role of immune cells in the vaccinated chickens, the bursectomized chickens were inoculated intravenously with the splenic, thymic, bursal and peripheral blood lymphocytes of the vaccinated chickens. Protective effects were shown in the chickens received the splenic cells and peripheral blood lymphocytes, indicating that these play an important role in eliciting the protective effects of the CA vaccine. From these results, it was found that the immune mechanism with CA vaccine against ILT involves mainly cell-mediated immunity.