Human serum paraoxonase (human PON1) has been shown to be important in the metabolism of phospholipid and cholesteryl ester hydroperoxides, thereby preventing the oxidation of low-density lipoprotein (LDL) and retarding atherogenesis. However, the exact substrate specificity of PON1 has not been established. In the present study we show that purified PON1 hydrolyses platelet-activating factor (PAF). We could find no evidence for contamination of our preparation with authentic platelet-activating-factor acetylhydrolase (PAFAH) by immunoblotting with a PAFAH monoclonal antibody or by sequencing the purified protein. In addition the specific PAFAH inhibitor SB-222657 did not affect the ability of PON1 to hydrolyse PAF (30.1+/-2.8 micromol/min per mg of protein with no inhibitor; 31.4+/-2.2 micromol/min per mg of protein with 100 nM inhibitor) or phenyl acetate (242.6+/-30.8 versus 240.8+/-31.5 micromol/min per mg of protein with and without inhibitor respectively). SB-222657 was also unable to inhibit PAF hydrolysis by isolated human high-density lipoprotein (HDL), but completely abolished the activity of human LDL. Ostrich (Struthio camelus) HDL, which does not contain PON1, was unable to hydrolyse PAF. These data provide evidence that PON1 may limit the action of this bioactive pro-inflammatory phospholipid.