The palladium (II) bis-chelate Pd (L 1 - 3) 2 and platinum (II) tetranuclear Pt 4 (L 4) 4 complexes of benzaldehyde thiosemicarbazone derivatives have been synthesized, and characterized by elemental analysis and IR, FAB (+) -mass and NMR (1 H, 13 C) spectroscopy. The complex Pd (L 2) 2 [H L 2 = m -CN-benzaldehyde thiosemicarbazone] shows a square-planar geometry with two deprotonated ligands (L) coordinated to Pd II through the nitrogen and sulphur atoms in a transarrangement, while the complex Pt 4 (L 4) 4 [H L 4 = 4 -phenyl-1-benzaldehyde thiosemicarbazone] has a tetranuclear geometry with four tridentate ligands coordinated to four Pt II ions through the carbon (aromatic ring), nitrogen, and sulphur atoms where the ligands are deprotonated at the NH group. The in vitro antitumor activity of the ligands and their complexes was determined against different human tumor cell lines, which revealed that the palladium (II) and platinum (II) complexes are more cytotoxic than their ligands with IC 50 values at the range of 0.07-3.67 μM. The tetranuclear complex Pt 4 (L 4) 4, with the phenyl group in the terminal amine of the ligand, showed higher antiproliferative activity (CI 50 = 0.07-0.12 μM) than the other tested palladium (II) complexes. Copyright © 2008 Wilfredo Hernándeza et al.