Purpose: The purpose of this study was to search activated genes that could be related to radiation adaptive response (RAR) induced by low-level radiation from 99mTc in human cell lines. Methods : We used gene discovery array (GDA) and representational difference analysis (RDA) methods. 99mTc-pertechnetate was added to 2 x 106/mL NC-37 cells (human lymphoblastic cells) to make concentrations ranging from 148 MBq/mL to 148 Bq/mL by serial 10 fold dilutions. After 44 hours, 2 Gy gamma irradiation was given to them using a Cs-137 cell irradiator. Results: As compared to the control (Con) group to which no 99mTc was added, those cells to which 148 and 14.8 KBq of 99mTc were added showed significantly lower damage to chromosomes, which was evaluated by metaphase analysis. Cells with 148 KBq 99mTc (T148 group) showed most significant protection. Activated genes in the T148 group as compared to Con group were evaluated by GDA and RDA methods. GDA revealed genes of casein kinase 2 (CK2) beta chain, immunoglobulins (Ig), human leukocyte antigen (HLA)-B, and two novel genes. Twenty RAR related clones were selected by RDA method. The size of those genes was from 234 to 603 base pairs. Conclusions: RAR was induced by low dose irradiation from 99mTc in NC-37 cell lines. Genes related to the response included CK2, Ig, HLA-B in human lymphoblastic cell lines. (Korean J Nucl Med 2001:35: 313-323)