Previously, mouse NIH 3T3 cells were stably transfected with human DNA polymerase β(β-pol) cDNA in the antisense orientation and under the control of a metallothionein promoter [Zmudzka, B.Z. and Wilson, S.H. (1990) Som. Cell Mol Gen ., 16, 311–320]. To assess the feasibility of enhancing the efficacy of chemotherapy by an antisense approach and to confirm a role for β-pol in cellular DNA repair, we looked for increased sensitivity to DNA damaging agents under conditions where β-pol is down-regulated in the antisense cell line. Such a sensitization is anticipated only where β-pol is rate-limiting in a DNA repair pathway. A number of agents were tested: cls -diamminedichloroplatinum II (clsplatin); 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU); lonizing radiation and the radio-mimetic drug bleomycin; the bifunctional alkylating agents nitrogen mustard and L-phenylalanine mustard (melphalan); the monofunctional alkylating agent methyl methane sulfonate (MMS) and ultraviolet (UV) radiation. In the cases of cisplatin and UV radiation, a significant enhancement of cytotoxicity was observed. Damage as a result of both of these agents is thought to be repaired by the nucleotide excision repair (NER) pathway. The results suggest that, in this cell line, β-pol is involved in and is rate-limiting in NER. We propose that down-regulation of β-pol by antisense approaches might be used to enhance the cytotoxic effects of cisplatin and other DNA damaging chemotherapeutic agents.