Hepatic assembly of triacylglycerol (TAG)-rich very low density lipoproteins (VLDL) is achieved through recruitment of bulk TAG (presumably in the form of lipid droplets within the microsomal lumen) into VLDL precursor containing apolipoprotein (apo)BHepatic assembly of very low density lipoproteins (VLDL) rich in triacylglycerol (TAG) 5 is initiated during and after translation and translocation of apolipoprotein (apo) B-100 across the endoplasmic reticulum membrane. The nascent VLDL particle is further enlarged in TAG content through a "second step" lipidation process, where a bulk TAG (presumably present within the microsomal lumen in the form of lipid droplet) is incorporated (1). Although much has been learned about the co-and post-translational lipidation of apoB-100, little is known about the nature or the dynamics of these lumenal lipid droplets (LLD) that are utilized as lipid precursor for VLDL assembly. Early studies with the abetalipoproteinemia gene MTTP suggested that the microsomal triglyceride transfer protein (MTP) is required for the partitioning of VLDL lipid precursor, mainly TAG, into the microsomal lumen in mouse liver (2), cultured murine primary hepatocytes (3), and the rat hepatoma McA-RH7777 cells (4). Thus, inactivation of MTP in the liver cells was associated with lack of LLD and invariably resulted in diminished assembly and secretion of VLDL. Recent proteomic and lipid characterization of LLD isolated from murine liver microsomes has shown that these lipid-rich entities are devoid of apoB but contain proteins such as TAG hydrolase, carboxylesterase 1, MTP, and apoE (5). Working with McA-RH7777 cells stably expressing recombinant human apoC-III, we have obtained evidence for the presence of a metabolically active TAG-rich entity within the microsomal lumen, which has a buoyant density resembling that of intermediate density lipoproteins (IDL) and low density lipoproteins (LDL)