Current thinking concerning the anti-cancer actions of platinum complexes view cis-platinum agents as active and trans-complexes as not being anti-neoplastic. It is generally accepted that the active cis forms exert their effects by binding to the DNA template. Data is presented here showing trans-Pt(NH3)2Cl2(trans-PDD) to be much more active than cis-Pt(NH3)2Cl2(cis-PDD) in inhibiting in vitro RNA transcription using both bacterial and eukaryotic enzyme sources. It is further shown that the polymerase enzyme is a much more sensitive target for these platinum agents than the DNA template under the conditions employed.