The structural organization and stability of apoB100 in complexes containing triglyceride (TG) and phospholipid have been examined. LDL was delipidated to form aqueous soluble apoB100-TG complexes that retain approximately 70% of LDL TG, but contain no other lipids. The apoB100-TG complexes exhibited reduced amphipathic alpha-helical content (17%) and net negative charge (-2.9 mV) as compared to native LDL-apoB100 (49% and -6 mV, respectively). Of 28 anti-apoB monoclonal antibodies tested, 15 showed partial or full reactivity with apoB100-TG. The immunoreactive epitopes of apoB100-TG were restricted to those situated in either the amino terminal globular domain (4 of 6) or in regions of apoB100 that are predicted to be composed of amphipathic beta-strands (11 of 13). Incubation of the apoB100-TG complex with palmitoyloleoylphosphatidylcholine (POPC) spontaneously (< 10 min) formed homogeneous lipoproteins (20 nm) that contained approximately 300 molecules of POPC per particle (apoB100-PC). Phospholipidation of apoB100-TG complexes partially recovered the alpha-helical content (34%) and net negative charge (-4.9 mV) of the native LDL and restored resistance of apoB100 to denaturation by guanidine HCl (5.8 M). Addition of phospholipids to apoB100-TG also increased the immunoreactivity of specific epitopes that are located primarily in regions of apoB100 that are thought to be constituted of amphipathic beta-strands. The effects of TG and phospholipid on apoB100 conformation appear to be highly domain-specific. On the basis of these results, we propose that the beta-strands of apoB100 may represent a nonflexible lipid-associating backbone, while the amphipathic alpha-helical domains may represent flexible lipid-binding regions that allow the particle to accommodate varying amounts of lipid.