Rat-liver cells (Ac2F cells) were transfected with cisplatin-damaged eucaryotic expression vectors carrying luciferase cDNA (cis-DDP-pVJ3luc) and incubated. The lysate of the incubated cells was subjected to a luciferase activity assay. Graded decrease in the activity was observed with increasing levels of platination of the plasmid DNA. In another experiment, Ac2F cells were pre-incubated in the presence of cisplatin ranging in concentration from 0.5 to 3 mu M. The viable cells were transfected with cis-DDP-pVJ3luc and incubated in the absence of this drug. The lysate of the incubated cells was subjected to the same assay. The level of the luciferase activity was raised with increasing cisplatin concentration. These results suggest that the repair activity for cis-DDP-pVJ3luc DNA is enhanced in Ac2F cells exposed to cisplatin.