Skin tumors induced in mice by chronic ultraviolet irradiation are highly antigenic and can induce a state of transplantation immunity in syngeneic hosts. In the present study, we compared the in vitro cytolytic activity of splenic lymphocytes from mice immunized with either a regressor or a progressor UV-tumor. The results of this comparison supported previous work implicating a role for tumor-specific cytolytic T lymphocytes in the rejection of regressor UV-tumors. The results also revealed that immunization with the progressor UV-tumor 2237 failed to elicit detectable levels of progressor tumor-specific CTL in animals capable of rejecting the immunizing tumor. Interestingly, following in vitro resensitization of both regressor and progressor immune spleen cells, we found a previously undetected lymphocyte population with anti-UV-tumor activity. Besides lysing UV-tumors in vitro, these lymphocytes also lysed a wide variety of additional tumor targets. This effector activity along with the analysis of cell surface markers indicated that these lymphocytes belong to that category of effector cells mediating natural-cell-mediated cytotoxicity (NCMC). As we had not detected cells with this activity in splenic lymphocyte preparations prior to in vitro resensitization, we examined lymphocytes from the local tumor environment during the course of progressor 2237 tumor rejection for either NCMC activity or tumor-specific CTL activity. This in situ analysis revealed lymphocytes exhibiting significant levels of cytolytic activity against several UV-tumors, thus implicating NK cells as effector cells in the rejection of progressor UV-tumors by immune animals. The mechanisms whereby NK cells with NCMC activity could be induced in immune animals are discussed in the context of class-II-restricted immune responses by helper/inducer T lymphocytes.