Platelet-activating factor (PAF) is a biologically active phospholipid synthesized by a variety of cells upon stimulation. The biological effects of PAF include activation of platelets, polymorphonuclear leukocytes, monocytes, and macrophages. PAF also increases vascular permeability, decreases cardiac output, induces hypotension, and stimulates uterine contraction. PAF has been implicated in pathological processes, such as inflammation and allergy. PAF is converted to the biologically inactive lyso-PAF by the enzyme PAF acetylhydrolase (PAF-AH). PAF-AHs are located intra- and extra-cellularly (e.g., cytosolic and plasma). Plasma PAF-AH is highly selective for phospholipids with very short acyl groups at the sn-2 position and is associated with lipoproteins. Recently, plasma PAF-AH has been linked to atherosclerosis and may be a positive risk factor for coronary heart disease in humans.