N-Ethyl and N,N'-diethyl derivatives (erythro- and threo-2-PtCl2; meso- and D,L-3-PtCl2) of [meso- and [D,L-1,2-bis(2,6-difluoro-3-hydroxyphenyl)ethylenediamine]dichloroplatinum(II) (meso- and D,L-1-PtCl2) were synthesized and tested for cytotoxicity on the estrogen receptor-positive (ER+) human MCF-7 breast cancer cell line. In this test, only D,L-1-PtCl2 and threo-2-PtCl2 showed strong cytotoxic properties. This revealed the existence of at least one NH2 fragment as a prerequisite for antitumor activity. Furthermore, studies on the three-dimensional structure of the new compounds demonstrated that the aryl and alkyl residues at the five-membered chelate ring have to be arranged in equatorial positions for the triggering of cytotoxic effects, very likely due to the reaction with d(GpG) sequences in DNA resulting in GG-N7,N7 chelates. A contribution of the ER-mediated processes--(a) hindrance of the cellular processing of Pt-modified DNA by overexpression of high mobility group domain proteins and (b) interruption of the vicious circle of mutual growth stimulation of breast cancer cells and granulocytes/macrophages by reduction of the formation of key cytokines--to the anti-breast cancer activity of threo-2-PtCl2 is unlikely, since we did not observe transcription activation in the test on ER+ MCF-7 breast cancer cells stably transfected with luciferase reporter plasmid ERE(wtc)luc.