We have studied several series of platinum-anthraquinone complexes in an attempt to understand their structure-activity relationships and to find out which features are important for their antitumor activity. The results of these studies reveal that very small changes in structure can lead to large changes in activity and that parameters which govern the activity in one series of compounds are of negligible importance in other series of very similar complexes. The activity of the Pt-anthraquinone complex is not the sum of activities of its components and tethering the most active Pt moieties to the most active anthraquinone ligands did not result in the most potent complexes. The platinum moiety binds covalently to the DNA while the anthraquinone exhibits several complex modes of interaction with the double helix. The activity of the platinum-anthraquinone complexes does not correlate with their ability to covalently modify DNA.