OBJECTIVE To explore the method of preparation of 99mTc labeled AntiVEGF UcAb 5-FU loaded polylactic acid nanoparticles (99mTC-5-FU-Ab-NPs),and investigate the biological distribution of the nanoparticles in human gastric carcinoma xenografts.METHODS Anti-VEGF monoclonal antibodyes (MCAB)in 5-FU-Ab-NPs were labeled with 99mTc using a modified Schwarz method. After isolation of the 99mTC-5-FU-Ab-NPs using a Sephadex G-250 column, the labeling percentage and radiochemical purity were determined using paper chromatography. The immunocompetence of the 99mTC-5-FU-Ab-NPs as tumor markers was determined using ELISA and immunohistochemistry. 99mTC-5-FU-Ab-NPs (experimental group), 99mTc-labelled murine multiclonal IgG loaded polylactic acid and nanoparticles (control group) were injected via the tail vein into SCID mice bearing human gastric carcinoma. A radio-immunity ECT image was developed at 2 and 6 h after the injection. Following the ECT imaging,the mice were sacrificed, their tissue and tumor radioactivity distribution determined, and percentage of the injected-dose per gram (%ID/g) and tumor/nontumor (T/NT) ratio calculated. High performance liquid chromatography (HPLC) was used to determine the 5-FU concentration in the tumor tissue and blood in the mice of both groups.RESULTS The percentage of 99mTC-5-FU-Ab-NPs labeling was 90%～95%. There was no obvious decrease in the antibody activity before and after labeling. The radio-immuno-imaging (RII) showed that the tumor image had developed 2 h after injection of the 99mTC-5-FU-Ab-NPs, and with time it was clearer at the 6th hour following the injection. The %ID/g of the tumor tissue at both 2 h and 6 h after the injection was significantly higher compared to the control group. The tumor %ID/g and the tumor to blood activity ratio (TB) of the experimental group at 6 h following the injection increased compared to that at 2 h, and at the same time, 5-FU concentration in the tumor of the experimental group continuously increased over time, and showed a significant difference compared to the 5-FU concentration in the tumor of the control group.CONCLUSION The 99mTc-5-FU-Ab-NPs prepared in this study are adequate to meet the demands of the RII, and the immune targeting ability of the anti-VEGF MCAB is reliable. Six hours after injection, the 99mTC-5-FU-Ab-NPs showed a relatively high specific concentration shadow in the human gastric carcinoma xenografts.