Abstract: Objective To improve the sensitivity of magnetic resonance molecular(MR) imaging on the basis of the targeting and magnifying effects of biotin-avidin system.Methods Biotinylated monoclonal antibody HAb18 was prepared,and the number of biotin molecules coupled to each antibody molecule was assayed together with the binding capacity of the biotinylated antibody. HAb18 was intravenously injected into 8 BALB/c nude mice bearing QGY-7723 tumor cells,followed by administration of 80µ g avidin as the chaser 24 h later and then Gd-DTAP-streptavidin injection after another 30 min. MR imaging was performed before and 10,30,60 min and 3,6,12,and 24 h after the injection of the contrast agents. All the images were obtained using SE T₁-weighted imaging sequence. The enhancement time course of the tumor,liver and muscles were determined by measuring the signal intensity (SI) in the region of interest in the tumor,and the enhancement ratio and contrast-to- noise ratio (CNR) of the tumor also calculated. Results On average,20 biotin molecules conjugated with each monoclonal antibody molecule,and the immunoactivity of the biotinylated antibody reached 91%. The SI of the tumor increased slowly and peaked at 6 h after injection of Gd-DTPA-streptavidin,when the enhancement ratio and CNR of the tumor were significantly higher then those measured on other time points. The SI of the liver reached the maximum value at 6 h after injection of the contrast agent,and then declined to the non-enhanced level 12 h after injection. The SI of the muscle exhibited no significant difference after enhancement.Conclusions In targeted MR of the tumor,biotin-avidin system produces specific and prolonged enhancement of the signals,but blood pool effect of Gd-DTPA-streptavidin also causes prolonged enhancement of the liver.