Cisplatin is among the most effective chemotherapeutic agents in the treatment of human ovarian cancer. The cytotoxicity of cisplatin results primarily from its ability to bind covalently to DNA and prevent DNA replication and transcription. The ubiquitin-proteasome pathway plays important roles in a broad array of basic cellular processes. Lactacystin is a selective inhibitor of the proteasome that can inhibit the ubiquitin pathway. However, the effect of lactacystin on DNA repair and the antitumor activity of cisplatin in ovarian cancer have not been evaluated. We report in this work that lactacystin, at concentrations that do not appear harmful, increased cisplatin toxicity in three resistant human ovarian carcinoma cell lines. In addition, lactacystin significantly enhanced DNA platination and decreased DNA repair of cisplatin-DNA adducts in these cell lines, as measured by atomic absorption spectrometry. Furthermore, Northem blot analysis and in vitro nuclear transcript elongation assay demonstrated that lactacystin dramatically reduced the steady-state mRNA expression and the rate of transcription of the DNA repair gene ERCC-1 in these cells. These observations indicate that proteasome inhibition has impact on nucleotide excision repair in several ways: i/ the normal ERCC-1 message upregulation is suppressed; ii/ cisplatin-DNA adduct repair is inhibited, and iii/ DNA platination, as well as cisplatin cytotoxicity, is enhanced.