The EJ-ras gene was placed under the transcriptional control of the steroid-inducible mouse mammary tumor virus promoter/enhancer and introduced into Rat-1 fibroblasts, yielding the 14C cell line. When these cells were exposed to dexamethasone in vitro, EJ-ras mRNA was induced 15- to 20-fold, the cells grew in agar, and, after injection of cells into syngenic Fischer 344 rats, they produced lethal fibrosarcomas. Inhibitors of poly(ADP ribose) polymerase, which prevent the activation of the purified enzyme by a synthetic octadeoxyribonucleotide duplex, inhibited both in vivo tumorigenicity and in vitro growth in soft agar. The enzyme inhibitor 1,2-benzopyrone, which was studied in detail, and other polymerase inhibitors had no effect on EJ-ras mRNA or p21 protein expression. Poly(ADP ribose) polymerase [NAD+:poly(adenosine diphosphate D-ribose) ADP-D-ribosyltransferase, EC 2.4.2.30] was inhibited by the drug in both untreated and dexamethasone-treated cells both in vitro and in vivo to the same extent, but biological consequences of enzyme inhibition were manifest only when the cells were in the transformed tumorigenic state.