Platelet-activating factor (PAF) is a bioactive phospholipid (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) synthesized by a variety of mammalian cell types. PAF induces hypotension, and activates neutrophils and platelets, among other actions. Removal of the acetyl moiety abolishes biological activity, so this reaction may regulate the concentration of PAF and its physiological effects. We have studied the significance of this reaction, which is catalyzed in vitro by an acetylhydrolase present in mammalian plasma, blood cells, and tissues. We have shown that the plasma PAF-acetylhydrolase is responsible for the degradation of PAF in whole human blood and that alternate pathways for PAF degradation in plasma or blood cells are negligible. Human plasma PAF-acetylhydrolase is associated with low and high density lipoproteins (LDL and HDL with apoE). We have confirmed that the activity is a stable component of these particles by density gradient ultracentrifugation, chromatography on heparin-agarose, and immunoprecipitation. The LDL-associated activity accounts for most or all of the PAF degradation that occurs in plasma ex vivo, while the HDL-associated activity contributes little to this process. However, the two activities likely are due to a single protein since the HDL- and LDL-associated PAF-acetylhydrolase activities can transfer from one lipoprotein to the other. These transfer processes are pH-dependent and specific, since they only occur from LDL to a well characterized subclass of HDL (apoE-containing HDL) and vice versa. We discuss the equilibrium between the two particles and the role that this process may have in vivo.