Platelet-activating factor-acetylhydrolase (PAF-AH), which removes the acetyl group at the sn-2 position of PAF, is distributed widely in tissues and plasma. Tissue cytosol contains at least two types of PAF-AH, isoforms Ib and II. Isoform Ib is a tertiary G-protein complex-like heterotrimeric enzyme that is involved in brain development such as formation of the brain cortex. Isoform II (PAF-AH(II)), however, is a 40-kDa monomer and has an amino acid sequence that exhibits a 41% identity with that of plasma PAF-AH. Although PAF-AH(II) preferentialy hydrolyzes oxidized phospholipids as well as PAF in vitro, the function of this enzyme has not, as yet, been elucidated. Here, we report that PAF-AH(II) functions as an anti-oxidant phospholipase. PAF-AH(II) was found to be an N-myristoylated enzyme that has never been reported among lipases and phospholipases. In MDBK cells treated with oxidants, PAF-AH(II) translocated from cytosol to membranes within 20 min, whereas in cells treated with anti-oxidants, it translocated, conversely, from membranes to cytosol. Overexpression of PAF-AH(II) in Chinese hamster ovary-K1 cells suppressed oxidative stress-induced cell death, which occurs by apoptosis. These findings suggest that intracellular PAF-AH(II) translocates between cytosol and membranes in response to a redox state of the cell and protects the cell against oxidative stress most probably by hydrolyzing oxidized phospholipids.