Investigations were performed to elucidate changes in normal syngeneic recipients after they received tumor-sensitized cells. Transfer of regional or nonregional lymph node cells or spleen cells from tumor-bearing inbred C3HeB/FeJ mice to normal syngeneic recipients caused the recipients to produced bone marrow, which mediated in vitro cytolysis of immunizing tumor target cells. This finding was not entirely limited to transfer of lymphoid cells. The transfer of myeloid cells, granulocytes, and cultured macrophages from tumor-bearing mice also produced cytotoxic cells. The extent of cytotoxicity following cell transfer was related to the duration of tumor growth in cell donors, to the degree of cytotoxicity, and to the number of cells transferred. Both the iv and ip routes were equally effective. Treatment of cells before transfer with trypsin or pronase, mitomycin C, or sublethal irradiation failed to prevent development of cytotoxicity in cells of the recipient, whereas freeze-thawing abolished this event. Serum from the cell recipient could inhibit the cytotoxicity demonstrated by lymphoid cells derived from the recipient or from a tumor-bearing animal. The findings indicate that "information" transferred to normal animals not previously exposed to a tumor results in production of tumor-specific cytotoxic cells in these animals.