Regulation of the assembly and secretion of apolipoprotein B-containing lipoproteins from liver cells is an extremely complex problem that is under investigation in numerous laboratories. During the past year, advances have been made in several relevant areas, including studies of the topography of apolipoprotein B as it translocates across the endoplasmic reticulum membrane, the regulation of translocation, the role of chaperons in this process, and the mechanisms governing the addition of core lipid to the nascent apolipoprotein B-containing particle.