Abstract: Objective To study the immunoactivity,biodistribution and metasbolic pattern of 131I-Herceptin in rabbits.Methods Herceptin was radiolabelled with 131I and its radiochemicalpurity(RCP) measured by size-exclusion high-pressure liquid chromatography(HPLC).The binding rate to BT-474 cells was measured to evaluate the immunoactivity of 131I-Herceptin.131I-herceptin(2.0 mCi/kg) was injected intravenously into New Zealand rabbits.Scintigraphy on emission computed tomography was performed at 3 h,1,3 and 5 days after injection,and the radiocounts of the heart,liver and kidney etc.were compared with that of the muscle to calculate the organ-to-muscle activity ratio(O/M).On the fifth day,the rabbits were killed and the blood,myocardium,lung and other organs were obtained for measuring the radiocounts on γ-counter to calculate the uptake percentage per gram tissue(ID%/g).Results The labeling rate of 131I-herceptin was 93% with RCP of 95% and binding rate to BT-474 cells of 36.9%.After injection of 131I-herceptin,the heart,lung and liver displayed dense radioactive regions but not the muscles and intestines.Three hours after injection,the O/M ratio of the heart was significantly higher than that of the lung,kidney and intestine(P<0.05),but decreased significantly one day after injection(t=10.817,P<0.001) with further decrement on days 3 and 5(P<0.05).The O/M ratio of liver on day 1,3,and 5 reduced significantly in comparison with that at 3 h(P<0.05).The uptake percentage was higher in the blood(11.3 ID/g%) than in the liver(2.8 ID/g%) and the myocardium(1.8 ID/g%).Conclusions 131I-herceptin possesses high immunoactivity which distributes mainly in the blood,liver and kidney,but with low uptake in the myocardium.