This study was designed to evaluate a direct method of Tc-99m labeling using β- mercaptoethanol as a reducing agent, and to investigate whether Tc-99m labeled specific monoclonal antibody against carcinoembryonic antigen (CEA-92) can be used for the scintigraphic localization of human colon cancer xenograft. Purified CEA-92 IgG was fragmented into F(ab')2 and then labeled with Tc-99m by transchelation method using glucarate as a chelator. Labeling efficiency, immunological reactivity and in vitro stability of Tc-99m CEA-92 F(ab')2 were measured and then injected intravenously into nude mice bearing human colon cancer (SNU-C4). Scintigrams were obtained at 24 hour after injection. Then nude mice were sacrificed and the radioactivity was measured. Labeling efficiency of injected Tc-99m CEA-92 F(ab')2, immunoreative fraction and in vitro stability at 24 hour of injected Tc-99m CEA-92 F(ab')2 was 45.2%, 32.8% and 57.4%, respectively. At 24 hour after injection, % ID/g in kidney (46.77) showed high uptake, but %ID/g in tumor (1.65) was significantly higher than spleen (0.69), muscle (0.16), intestine (0.45), stomach (0.75), heart (0.48) and blood(0.45). There was no significant difference between tumor and liver (1.81). Tumor contrast as quantitated by tumor to blood ratio of Tc-99m CEA-92 F(ab')2 was increased significantly (p〈(0.005) until 24 hours (3.70), and there was no statistical difference from tumor to blood ratio of I-131 CEA-92 F(ab')2. The scintigram demonstrated localization of radioactivity over transplanted tumor, but significant background radioactivity was also noted over kidney and abdomen. It is concluded that CEA-92 F(ab')2 can be labeled with Tc-99m by a direct transchelation method using β- mercaptoethanol as a reducing agent and Tc-99m labeled CEA-92 F(ab') can be used for the scintigraphic localization of human colon cancer xenograft in nude mice model.