The purpose of this study was to establish mononuclear cell cultures such as lymphocytes or buffy coat for the biological dosimetry of in vitro irradiation of the radionuclide Tc 99m in order to exclude the effect of residual doses seen in the cultures of whole blood. Biological dosimetry of Tc-99m on cultured mononuclear cells at doses ranging from 0.05 to 6.00 Gy, by scoring unstable chromosomal aberrations(Ydr) observed in cultured lymphocytes, were performed using peripheral venous blood of healthy normal person. The results showed that; (1) Jn vitro irradiation of radioisotope in separated lymphocyte or buffy coat showed trace amount af residual doses of isotope after washing. Residual doses of isotopes are increased in proportion tn exposed time and irradiated dose without difference between I-131 anct Tc 99m. (2) We obtained these linear-quadratic dose response equations in lymphocyte and buffy coat culture after in vitro irradiation of Tc-99m, respectively (Ydr = 0,001949 D+ 0,006279B+ 0.000185; Ydr= 0.002531 D 0.003274 D+0.003488). In aonclusion, the linear quadrstic dose response equation from in vitro irradiation of Tc 99m with lymphocyte and buffy coat culture was thought to be useful for assessing Tc 99m indueed biological effects. And mononuclear cell cultures seem to be the most appropriate experimental rnodel for the assessment of biological dosimetry of internal irradiation of radionuclides.