Monoclonal antibody 3H11 was labelled with99mTc by modified Schwartz method. The antibody was incubated in a glass test tube at room temperature for 30min with a 1000-fold molar excess of 2-mercaptoethanol. The mean labelling efficiency of 3H11 with99mTc was more than 95%. The immuonreactivity of99mTc-3H11 was more than 80% by ELISA's method. Competetion results in vitro and HPLC analysis showed that99mTc was combined at the high affinity sites of antibody. The biodistribution in nude mice bearing 823 gastric cancer xenograpfts showed that the radioactivity in tumor at 24h postinjection was the highest except for that in kidney. The tumor uptake was 8.98±2.42% i.d/g. The ratio of tumor to blood was over 1.5 and that of tumor to liver was more than 2.5 at 24h post-injection. The tumor was clearly imaged at 22h postinjection. The inital clinical results showed that99mTc-3H11 was stable in vivo and was good located at the lesion sites.