High affinity complexes for the tumor were obtained by changing pH and composition in the preparation of Tc- 99m-DMSA. The purpose of this study was to investigate the tumor affinity, and in vitro and in vivo characteristics of these complexes. The results obtained were as follows; 1) Tumor imaging agent was formed successfully at pH 6.0∼9.0 and renal imaging agent at pH 2.0∼5.0. 2) The serum protein binding of Tc-99m-DMSA was 89.1∼92.8% at pH 2.0∼5.0 and 11.8∼30.5% at pH 6.0∼9.0 respectively, and it was not changed with time. 3) The T 1/2 of tumor affinity complex in blood between 3 and 6 hours after injection was 187±29 minutes (mean±SD). 4) In the blood, the radioactivity was mainly in the plasma, and less than 1% was in the cellular components. 5) In the Walker carcinosarcoma 256 bearing Wistar rats, the radioactivity in the kidney increased, and decreased in the skeleton with time. The radioactivity in the tumor showed the peak in 6 hours after injection and decreased thereafter. 6) In the tumor cell, the radioactivity localized mainly in the cytosol, the soluble fraction of the cytoplasm. This study provides the basic knowledge about tumor affinity and usefulness of Tc-99m-DMSA in the diagnosis of malignant disease.