Donor allogeneic C57Bl/6 lymphoid cells from peritoneal cavity, lymph nodes, thymus and spleen of immunized and non-immunized mice were used for adoptive immunotherapy of L-1210 ascites leukemia in DBA/2J recipients. Donor effector cells were injected i.p. into leukemic mice which were given a whole-body irradiation in a dose of 300 r X-rays prior to leukemia inoculation. The in vitro cytotoxicity of the effector cells was tested by the LT- and 51Cr-tests. Cytokinetics of leukemia undergoing therapy was followed by impulse-cytophotometry. An adoptive chemo-immunotherapy with the aid of peritoneal or splenic lymphoid cells from immunized donors proved to be most effective. The cells were injected on the second day into recipients inoculated with leukemia on the day 0, and then given on the first day a single i.p. injection of cyclophosphamide in a dose of 50 mg/kg body weight. Under these conditions the secondary disease did not influence therapy and apart from a distinctly prolonged survival there was also noted a number of permanent survivals of leukemia. It was also found that peritoneal and splenic lymphoid cells of immunized donors exhibited most pronounced direct and indirect cytotoxicity against target cells in vitro. In vivo, the effector cells mounted an attack on the L-1210 cells probably at the onset of the G1-phase of the cell cycle.