It has been previously shown that the BALB/c lymphoma YC8 is susceptible to lysis by syngeneic anti-DBA/2 lymphocytes and that YC8-bearing BALB/c mice can be cured by adoptive transfer of such immune effectors. In this study in vivo and in vitro functions of the curative immune lymphocytes have been evaluated together with the role of the host immune system in the mechanism of tumor eradication. It was found that the curative anti-DBA/2 lymphocytes were not directly cytotoxic to YC8 cells although they developed into YC8-lytic cells after in vitro restimulation with YC8. In vivo, the immune lymphocytes were able to mediate a tumor-specific delayed type hypersensitivity reaction against YC8 but had a low tumor-neutralizing activity in the Winn assay. Proliferation of infused BALB/c anti-DBA/2 lymphocytes was necessary for the in vivo therapeutic effect, since irradiation of effector cells or treatment of the donor immune lymphocytes with vinblastine abolished their curative capacity. Immunodepression of the T cell compartment of the prospective tumor-bearing animals by thymectomy plus irradiation or its abrogation in B mice (thymectomized, lethally irradiated, and reconstituted with fetal liver cells) did not interfere with the therapeutic effect of the transferred anti-DBA/2 lymphocytes. Blocking the macrophage functions of the host by carrageenan, however, abolished the therapeutic effect of immune lymphocytes. These data indicate that a radiation-resistant, non-T cell is involved in the tumor eradication induced by anti-DBA/2 lymphocytes. It was also shown that cured mice, tested 90 days after therapy, become resistant to 5 X 10(3) LD80 YC8 cells and that this resistance was due to the presence of memory cells derived from the transferred and not from the host lymphocyte population.