A number of new Pt(II) complexes is described having the general formula PtCl(2)(LL), where LL is a chelating diamine ligand. Ligands LL were chosen as D,L-2,3-diaminopropionic acid and its ethyl ester, and D,L-2,4-diaminobutyric acid and its ethyl ester. The compounds were characterized using analytical and spectroscopic methods. The influence of the size of the chelate ring and its functionalization on the biological properties was studied. It was demonstrated by circular dichroism (CD) that the effects on the secondary structure of DNA induced by the four complexes are different. The interaction takes place at the N7 position of the purine bases, as shown by NMR studies. The platinum complexes of 2,3-diaminopropionic acid and 2,4-diaminobutyric acid are able to form intrastrand adducts with DNA and to distort the double helix by changing the base stacking. The ethyl ester derivatives uncoil the DNA from the B form to the C form. The interactions with 5'-GMP and DNA were compared with their antitumor activity. The platinum complexes of diaminocarboxylic acids exhibit cytotoxic activity in the A431, HeLa, and HL-60 cell lines in a dose- and time-dependent manner.