Purpose: The purpose of this study was to compare the radiation adaptive response (RAR) in peripheral lymphocytes (PL) of patients induced by Tc.-99m MDP and Tc-99m DTPA scintigraphies. Materials and Methods: Lymphocytes from 45 patients (25 males, 20 females, mean age 44±18 years) were collected before and after scintigraphies using 740 MBq Tc-99m MDP (n=22) or Tc-99m L)TPA (n=23). Lympho-cytes from 20 controls (12 males, 8 females, mean age 43±7 years) were also callected. They were exposed to challenge dose of 2 Gy γ-rays using a Cs-137 cell irradiator, Number of ring-form (R) and dicentric (D) chromosomes was counted under the light microscope. From them a representative score, Ydr, was calculated as Ydr=(D+R)/cells. Adaptation index (AI) was defined as difference of Ydr between unconditioned and conditioned lymphocytes. Ydr was also measured after an administration of cyclohexi-mide (CHM), a protein synthesis inhibitor, before challenge dose. Results: RAR was induced in both groups of patients. CHM abolished the adaptive response in both groups. AI of Tc-99m MDP group was significantly higher than that of Tc-99m DTPA group. Conclusion: Tc-99m MDP induced RAR was more prominent than those induced by Tc-99m DTPA. (Korean J Nucl Med 2000;34:252-9)