A clinical trial of radioimmunoguided surgery (RIGS) using 131-I labeled monoclonal antibody (MAb) 3H11 was performed on 25 cases with gastric cancer in this study. The results showed that the route of the administration of radiolabeled MAb is the key point of successful RIGS for gastric cancer. The RIGS in 19 cases in the group of submucosal injection via endoscope was very satisfying. For the detection of metastatic lymph nodes, the sensitivity rate of RIGS was 99.2% (118/119), the specificity, 97.7% (43/44), and the accuracy 98.8% (161/163). The accuracy rate in detecting tumor infiltration of the gastric wall was 95.9% (94/98), the sensitivity 94.6% (35/37), and the specificity 96.7% (59/61). Thus, the use of RIGS may improve the radical resectability rate and possibly the overall survival rate in patients with gastric cancer.