Human milk contains numerous factors such as immunoglobulins, lactoferrin, lysozyme, macrophages, etc., which serve an immunoprotective role. Platelet-activating factor (PAF) is one of the most proinflammatory agents thus far described. PAF is metabolized to the biologically inactive lysoPAF by the enzyme PAF-acetylhydrolase (PAF-AH). In the present study we have demonstrated that PAF-AH activity is present in human milk. The activity was associated with aqueous phase and was not stimulated by the addition of bile salts or Ca2+. The activity of PAF-AH in human milk was not affected by the addition of propranolol or NaCl. PAF, and 1-acyl-2-acetyl-glycerophosphocholine were the only substrates cleaved by the enzyme. Based on these properties it is concluded that the milk PAF-AH is not the lipoprotein or bile salt-stimulated lipase known to be present in milk. Inhibitor studies revealed that the enzyme in human milk was the plasma type PAF-AH. The activity of PAF-AH was stable at pH 4.0 at 37 degrees C and the activity varied in milk samples obtained from various species. The enzyme was secreted by milk macrophages. The presence of PAF-AH in human milk may explain, in part, the beneficial effects of breast feeding in the prevention of necrotizing enterocolitis by inactivating the potent proinflammatory autacoid, PAF.