ATP-binding cassette (ABC) transporter ABCG1 (ABC8) is induced in monocyte-derived macrophages during cholesterol influx mediated by acetylated low-density lipoprotein. Conversely, lipid efflux in cholesterol-laden macrophages, mediated by the cholesterol acceptor HDL(3), suppresses the expression of ABCG1. Immunocytochemical and flow cytometric analyses revealed that ABCG1 is expressed on the cell surface and in intracellular compartments of cholesterol-laden macrophages. Results demonstrate a regulator function for ABCG1 in cholesterol and phospholipid transport define a biologic activity for ABC transporters in macrophages (1). ABC transporters belonging to the ABCG (White) subfamily (ABCG1, ABCG5, and ABCG8) have been shown to be critically involved in the regulation of lipid-trafficking mechanisms in macrophages, hepatocytes, and intestinal mucosa cells (2) Studies show that treatment with Rhodobacter sphaeroiders LPS, an LPS antagonist, or PD169316, a specific p38 MAP kinase inhibitor, prevented the induction of ABCA1 by LPS. Therefore, this suggests that both transport of LPS from the plasma membrane to an intracellular site and activation of p38 MAP kinase are involved in the LPS-mediated induction of ABCA1 (3).