Gemcitabine is a new standard chemotherapeutic agent used in the treatment of pancreatic cancer, but the mechanisms of gemcitabine sensitivity are still controversial. In our study to determine a mechanism that regulates gemcitabine sensitivity, we carried out molecular analysis on the susceptibility of the pancreatic cancer cells. Using a gemcitabine-sensitive pancreatic cancer cell line KLM1, we established a resistant cell line KLM1-R exhibiting a 20-fold IC50-value (the concentration of gemcitabine causing 50% growth inhibition). Microarray analysis of genes showed specific expression of selenoprotein P, one of the anti-oxidants, in the KLM1-R cell line but not in the KLM1 cell line. Administration of selenoprotein P inhibited the gemcitabine-induced cytotoxicity in the pancreatic cell lines. The levels of intracellular reactive oxygen species (ROS) were increased in the KLM1 cells by gemcitabine, but selenoprotein P suppressed the gemcitabine-induced ROS levels. Furthermore interferon-γ suppressed the expression of selenoprotein P mRNA and increased intracellular ROS level, leading to the recovery of the gemcitabine sensitivity in KLM1-R. These results suggest a novel mechanism that selenoprotein P reduces the intracellular ROS levels, resulting in the insusceptibility to gemcitabine. © 2004 Wiley-Liss, Inc.