b 2 -Glycoprotein I (b 2 -GPI), also known as apolipoprotein H, is a plasma glycoprotein with poorly defined gene regulation. The aim of this study was to clarify the role of oxidative stress in b 2 -GPI gene regulation and determine the essential transcription element regulating b 2 -GPI expression. We demonstrate that expression of b 2 -GPI at the protein and mRNA levels was significantly elevated in Huh7 and HepG2 cells treated with 100 mM hydrogen peroxide (H 2 O 2 ). To address the transcriptional mechanism of H 2 O 2 -mediated b 2 -GPI gene regulation, several promoter constructs were cloned and characterized by deletion assays. A region spanning from À2141 to À1419 (relative to the transcription start site), which contains two activator protein-1 (AP-1) sites (AP1-2 and AP1-3) and one nuclear factor-kappaB (NF-kB) site was found to be the main target site for up-regulation of b 2 -GPI promoter activity by oxidative stress. In addition, we found that H 2 O 2 stimulation enhanced the nuclear translocation of AP-1 and NF-kB subunits. Using an electrophoretic mobility shift assay, it was confirmed that nuclear protein binding to the AP1-2, AP1-3, and NF-kB sites was increased in Huh7 cells treated with H 2 O 2 . Knockdown of the c-Jun, c-Fos, p65, and p50 genes using small interfering RNAs (siRNAs) further confirmed that AP-1 and NF-kB play an essential role in the H 2 O 2 -induced b 2 -GPI expression. Overall, these findings provide new insight suggesting that multiple cis-elements in the b 2 -GPI promoter work cooperatively to regulate b 2 -GPI expression in cells under oxidative stress.