The distribution of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine: acetyl hydrolase acetyl hydrolase activity between different types of human plasma lipoproteins was studied. It was found that lipoprotein-depleted plasma is practically devoid of acetyl hydrolase and of almost all acetyl hydrolase activities recovered in the plasma lipoprotein fraction. Among different types of plasma lipoproteins the bulk of acetyl hydrolase is bound to low density lipoproteins; of those not more than 5-10% is associated with high density lipoproteins. Isolated plasma high density lipoproteins do not influence the activity of acetyl hydrolase associated with low density lipoproteins. It is suggested that low and high density lipoprotein acetyl hydrolase may play different functional roles in human plasma.