Cultures of chicken lymphoid tissues were tested for their capacity to lyse (51)Cr-labeled chicken, burro (BRC), and human red blood cells (HRC) in the presence of phytomitogens. PHA-stimulated cultures lysed all three types of targets, while PWM and Con A showed a "target cell specificity" for HRC and BRC, respectively. In mixtures of target cells only the appropriate targets were lysed by lymphocytes activated by either Con A or PWM indicating that soluble lymphotoxins do not play a major role in these reactions. Preincubation experiments suggested that there may be a population of pre-existing aggressor cells which only require linking to the targets by the mitogens for activation of their cytotoxic potential. Strong cytotoxic reactions were found with spleen cells, peripheral blood leucocytes, and bone marrow cells. Thymocytes were less active but could be stimulated for significant cytotoxicity, while bursal cells were generally unreactive. Spleen cells from agammaglobulinemic chickens totally lacking serum immunoglobulins and B cells with surface-bound immunoglobulins were as active as cells from normal chickens. The activity of spleen cells, from which phagocytic cells were removed was also unimpaired. These results indicate that the development of cytotoxic effector lymphocytes in mitogen-treated leucocyte cultures is a property of T lymphocytes. Although bone marrow cells fail to proliferate in response to these phytomitogens, they do have strong cytotoxic reactivity suggesting that different subsets of thymic-derived lymphocytes are responsible for mitogen-induced transformation and mitogen-induced cytotoxicity.