Highly specific anti-human colorectal carcinoma monoclonal antibody(A7) was developed by fusion of mouse myeloma cells with mouse spleen cells immunized by colon cancer cells. Neocarzinostatin (NCS) was bound to A7 preserving both antibody and drug activities. This conjugate (A7-NCS) was applied for clinical trial. No serious side effects were reported, and half of the eight patients with metastatic liver tumor responded to A7-NCS well. To overcome the variety in the expression of tumor-specific antigen on tumor cells, new types of conjugates were developed. Mitomycin C(MMC) was bound to Dextran sulphate. And this conjugate (M MC-Dex) was bound to A7 with the expectation that MMC would release from Dextran that was delivered to the surface of the cancer cells. This type of conjugate would be effective not only against cancer cells that express antigens detected by A7 but also against any cells around cancer cells detected by A7. The possibility and problems in cancer therapy using immunotoxin are discussed.