The bioactive lipid lysophosphatidylserine (LPS) is postulated to induce important biological responses and to be produced by phosphatidylserine-specific phospholipase A 1 (PS-PLA 1 ). To evaluate the functional roles of LPS in vivo, a facile assay method for PS-PLA 1 has been awaited.Recombinant human PS-PLA 1 was produced using a baculovirus system, and anti-human PS-PLA 1 monoclonal antibodies were generated. Two clones were then selected for a 2-site immunoassay. The resulting PS-PLA 1 assay reagent was applied to a commercial automated immunoassay analyzer.Satisfactory results were obtained for the within-run and between-run precision, interference, detection limit, and linearity of this PS-PLA 1 assay. The mean±SD of the serum PS-PLA 1 antigen concentration in the 191 healthy subjects was 33.8±16.6µg/l, and the central 95th percentile reference interval for the serum PS-PLA 1 antigen concentration was 13.8–74.1µg/l. The concentration was significantly (p<0.001) higher among men (13.8–80.6µg/l) than among women (12.1–68.8µg/l). We did not find a correlation between PS-PLA 1 and existing laboratory tests.The present PS-PLA 1 assay method can be applied to clinical laboratory testing, and further studies are warranted to establish its clinical significance.