Chloroethylnitrosoureas have been used widely to treat human and experimental animal tumors. We have earlier observed that greater than 90% of the mice transplanted with syngeneic tumors survive following treatment with nitrosoureas such as 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and furthermore, they resist subsequent challenge with the same tumor. The present investigation was initiated to determine the mechanism by which BCNU brings about this effect. Treatment of tumor cell targets in vivo or in vitro with BCNU, increased their susceptibility to macrophage (M luminal diameter)-mediated cytotoxicity as measured in a direct cytotoxicity assay or in an antibody-dependent cell-mediated cytotoxicity (ADCC) assay. In contrast, the antitumor cytotoxicity caused by cytotoxic T lymphocytes (CTL), natural killer (NK) cells, or lymphokine-activated killer (LAK) cells, was not altered following BCNU treatment of tumor targets. Studies were also conducted to investigate the direct effect of BCNU in vivo on various cytotoxic effector cells. For this purpose, M luminal diameter, NK, LAK, and CTL activities from BCNU-treated-tumor-bearing mice were screened for cytotoxicity against untreated tumor targets in vitro. It was observed that tumor-specific CTL and LAK cell activity increased in BCNU-treated tumor-bearing mice when compared to untreated controls while the cytotoxic potential of NK cells and M luminal diameters was not altered. The present study suggests that antitumor drugs such as BCNU are not only tumoricidal but also selectively act in a variety of ways at both the effector and target cell level, leading to overall enhanced antitumor immunity and high rate of cures from the syngeneic tumor challenge.