The validity of ^99mTc-YIGSR, a novel receptor radio-tracer, in imaging the Ehrlich ascites tumor was evaluated. YIGSR, a pentapeptide of laminin, was labeled with ^99mTc by using a bifunctional chelator S-Acetly-NH3-MAG3. The MIBI was labeled with ^99mTc by following the kit instruction. The mice of tumor group were intravenously injected 1-2 mCi of ^99mTc-YIGSR or ^99mTc-MIBI via caudal vein, immobilized and imaged under a Gamma camera. The same procedure was performed in mice of blockade group, in which the unlabeled YIGSR was previously injected to block the receptor-recognition sites, and inflammation group serving as control. The reverse-phase Sep-Pak C18 chromatogram was found to have an essentially complete conjugation between YIGSR and S-Acetly-NH3-MAG3. The conjugated YIGSR could be radio-labeled successfully with ^99mTC at room temperature and neutral pH, with a radio-labeling yield of 62%. Without the chelator S-Acetly-NH3-MAG3, the YIGSR was labeled with ^99mTc at an efficiency of 4%. The imagological study revealed obvious tumor accumulation of ^99mTc-YIGSR 15 min after the injection, and the uptake peaked after 3 h with a tumor-to-muscle ratio （T/M） of 11.36. The radio-tracer was slowly cleared up and resulted in a T/M of 3.01 at the 8th h after the injection. As for blocked group, the tumor uptake of radiotracer was significantly lower, with the highest T/M being 4.61 after 3 h and 0.89 after 8 h. The T/M was 3.72 at the 3rd h and 1.29 at the 8th h after the ^99mTc-YIGSR injection in the inflammatory group. The T/M was significantly higher in tumor group than in inflammatory group or control group （P〈0.001）. In the ^99mTc-MIBI group, the T/M was 1.40 at the 3rd h and 0.55 at the 8th h after the injection, which showed a significant difference as compared with ^99mTc-YIGSR （P〈0.001）. It is concluded that YIGSR can be successfully radiolabelled by using S-Acetly-NH3-MAG3. ^99mTc-YIGSR has many advantages in tumor imaging, such as quick and clear visualization, high sensitivity and sp
