A cytotoxic lymphocyte assay, using cells that adhered to plastic as the target cells and neutral red as the indicator for lysis, was applied to chickens infected with either infectious bronchitis virus or fowl pox virus. Both target and effector cells were derived from the same bird. Cytotoxic lymphocytes were generated in birds infected with either virus. The activity was confined to cells of the spleen after initial immunisation, but could be detected in white cells from the blood after challenge at a peripheral site, with both virulent and avirulent virus strains. It is likely that the cytotoxic cells are T-lymphocytes. The cytotoxic assay system used was an economical and convenient method for chickens which overcame the need for inbred lines of birds.