The monoclonal antibody (MAb) RWS4 specific to membrane-associated antigen of human gastric adenocarcinoma was purified by protein A-Sepharose 4B affinity chromatography and labeled with 131I by chloramine-T method. 131-RWS,, was injected (65 μCi/10μg/0.2 ml, intraperitoneally) into the stomach cancer-bearing nude mice (solid tumor about 1 cm in diameter), and its biodistribution was studied by SPECT and gamma-counter over a peroid of 7 days. A clear image of transplanted tumor was observed on the 4th day, and the image became more clear on the 6th day. After SPECT scanning, the animals were killed on the 3rd to 7th day separately and radioactivity was detected in various organs. The ratios of T/NT were calculated. The results were shown as follows: tumor/blood, was 3.41±0.29 on the 6th day and the tumor/other organs (liver, spleen, stomach, lung, heart, kidney and brain etc.) were>3. The specificity of the 131I-RWS4 was 7.74±0.65. The monoclonal antibody (MAb) RWS4 specific to membrane-associated antigen of human gastric adenocarcinoma was purified by protein A-Sepharose 4B affinity chromatography and labeled with 131I by chloramine-T method. 131-RWS,, was injected (65 μCi/10μg/ 0.2 ml, intraperitoneally) into the stomach cancer-bearing nude mice (solid tumor about 1 cm in diameter), and its biodistribution was studied by SPECT and gamma-counter over a peroid of 7 days. A clear image of transplanted tumor was observed on The 4th day, and the image became more clear on the 6th day. After SPECT, the animals were killed on the 3rd to 7th day separately and radioactivity was detected in various organs. The ratios of T/NT were calculated. The results were As shown: tumor/blood, was 3.41±0.29 on the 6th day and the tumor/other organs (liver, spleen, stomach, lung, heart, kidney and brain etc.) were> 3. The specificity of the 131I-RWS4 Was 7.74±0.65.