Therapeutic effect of an alkylating agent, carboquone (CQ: Esquinon, 1mg/vial Sankyo Co., Tokyo) was investigated with a mouse mammary tumor MM-46 inherited the MM-antigen. MM-46 tumor cells (3 X 10(6)) were implanted subcutaneously into the axillary region of syngeneic C3H/He mice and CQ was administered intravenously at a dose of 2 mg/kg by a single injection on day 1, 4, 7 or 10. The experiment was carried out with 10 mice in each group. As a result, treatment of CQ elicited a complete regression effect on MM-46 tumor; the rate of complete regression in the group treated with CQ on day 4 or day 7 was 10/10, but that in control group was 0/10. Administration of CQ after the tumor inoculation brought about necrosis of the subcutaneous tumor, followed by complete regression and only a scar remained by day 25 at the site of tumor exclusion. In this experiment no recurrence or metastasis was observed after tumor regression. Since the mice which rejected the once developed tumor were resistant to them on rechallenge with the same amount of MM-46 tumor cells, it was suggested that an immunological response would play an important role in exclusion of the tumor. The complete regression effect described above has generally been reported on the antitumor pattern of the host-mediated agents such as polysaccharides of Basidiomycetes, while has hardly on that of the alkylating agents except cyclophosphamide. The mechanism of such the regression effect attributed to the activity of CQ is obscure, but these results suggest that this phenomenon may be dependent on the inherent antigenicity of MM-46 tumor.