Cisplatin (cis-diamminedichloroplatinum(II); cis-DDP) is used as an effective drug for treatment of a variety of cancers, such as carcinomas of bladder, ovarian, and testicular origin. Immunopurified DNA polymerase-alpha from rat prostate tumor PA-3 cells was inhibited (50%) in the presence of cis-DDP (165 microM) and PtCl2(en) (cis-dichloroethylenediamine platinum (II); DEDAP) (75 microM) and remained uninhibited in the presence of trans-DDP. Immunopurified DNA polymerase-alpha was preincubated with cis-DDP and separated from unreacted cis-DDP by gel filtration chromatography. The platinated DNA polymerase-alpha was unable to initiate the DNA chain extension reaction. N-ethylmaleimide (1 mM), a thiol group modifier, also inhibited (95%) the DNA polymerase-alpha catalyzed reaction in vitro. Possible disruption of a zinc-finger motif of the DNA polymerase-alpha polypeptide chain by replacement of zinc is suggested.