Using a 3H-proline release assay, in vitro cell-mediated cytotoxic (CMC) response of spleen lymphocytes from B13B13 chickens infected with Marek's disease (MD) virus (MDV) was investigated during the course of infection. Syngeneic MD-specific anti-tumoral CMC and natural killer (NK) activity was compared from one to six weeks after infection. A parallel increase in the two types of CMC responses were demonstrated and effector cells with similar characteristics seemed to be involved. Anti-thymocyte serum (ATS) treatment and depletion of macrophage-like cells, using plastic adherence or carbonyl iron treatment, slightly reduced the CMC response against both types of target cells in MDV-infected chickens as well as in controls. Nevertheless, even after depletion treatment, a higher cytotoxic activity was maintained in MDV-inoculated chickens. The involvement of specific antibodies was excluded. It was concluded that the inoculation of the HPRS-16 strain of MDV in B13B13 chickens increased a splenic NK-like activity against lymphoblastoid cell lines, wether MDV-transformed or not. In chickens vaccinated with HVT (Herpes virus of turkey) at hatching, an increase of NK-activity was observed earlier, ie two weeks after MDV inoculation.