Oxidized low-density lipoprotein (oxLDL) particles contain low molecular weight species which promote the differentiation of monocytes and activate polymorphonuclear leukocytes. One of these substances was recently isolated and purified from oxLDL, and identified as azelaoyl PC. Butanoyl PAF is a closely related compound which retains at least 10% of the agonist potency of platelet-activating factor (PAF) itself. Further, butanoyl PAF is present in oxLDL in amounts more than 100 times greater than enzymatically generated PAF. Butanoyl PAF is therefore one of the important signalling molecules present in oxLDL.