Background We previously reported that iodine-131(131I)-labeled anti-pro-gastrin-releasing peptide (ProGRP(31-98)) monoclonal antibody D-D3 could selectively accumulate in the tumor sites of nude mice bearing small cell lung cancer (SCLC) xenografts.However,131I-D-D3 was cleared slowly from the body,and the best radioimmunoimaging time for SCLC was 72-96 hours after injection.The aims of this study were to radiolabel anti-ProGRP(31-98) D-D3 monoclonal antibody with technetium-99m (99mTc) and to investigate the biodistribution of this antibody in healthy ICR mice.Methods D-D3 was labeled with 99mTc via the 2-mercaptoethanol reduction method.99mTc-D-D3 was purified by the gel column separation method.The labeling efficiency and radiochemical purity were measured by thin-layer chromatography.The immunological activity of 99mTc-D-D3 was determined with cell conjugation assays.99mTc-D-D3 was injected into healthy ICR mice via a tail vein,and all the healthy ICR mice were sacrificed by cervical dislocation at a designated time.Then,the blood and major organs were removed and weighed,and counted in a gamma scintillation counter to determine the percentage of the injected dose per gram (％ID/g).Results The labeling rate and the radiochemical purity of 99mTc-D-D3 were (73.87±2.89)％ and (94.13±4.49)％,respectively.The immunobinding rates of 99mTc-D-D3 to the human small cell lung cancer NCl-H446 cell line and lung adenocarcinoma A549 cell line were (81.2±2.37)％ and (24.3±1.46)％,respectively.The distribution data of normal ICR mice demonstrated that 99mTc-D-D3 was mainly distributed in the liver,kidney and lung,and less in the brain tissue and muscle.Conclusions 99mTc-D-D3 antibody not only had high radiochemical purity,but also had good stability both in vitro and in vivo,and maintained good immunological activity.99mTc-D-D3 was metabolized mainly in the kidney and liver,and the blood radioactivity decreased rapidly.Thus,99mTc-D-D3 is conducive to the radioimmunoimaging of SCLC.