The proposal of this work was to investigate the effect of the radioactive complex of platinum(II) with tetracycline, [PtCl 2 (C 22 H 24 N 2 O 8 )]*, or [Tc-Pt(II)]*, on K562 cells-blood human cells of leukemia-and verify if the internal radio-chemotherapy would be able to produce additional effects compared with the non-labelled complex, [PtCl 2 (C 22 H 24 N 2 O 8 )], or [Tc-Pt(II)]. The concentration required to inhibit 50 % of cellular growth, (IC 50 ), was 2.5 ± 0.2 lM for [Tc-Pt(II)]* and 14.5 ± 0.9 lM for the non-labelled molecule [Tc-Pt(II)]. This result suggest that the [Tc-Pt(II)]* could be a potent radiosensitizer evoking a supra additive effect. Treatment using the internal radiochemotherapy may be a useful alternative to reduce the drug concentration required for effective inhibition of the tumor growth.