The binding maximum of apoA-1 (N) in triolein (TO)-egg yolk phosphatidylcholine (PC) emulsions was 10-fold larger than that in PC large unilamellar vesicles (LW) of similar size (100 nm) with no significant difference in the affinity. Replacement of the long-chain triglyceride, TO, by medium-chain triglycerides or cholesteryl oleate in emulsion cores significantly decreased the N value. The C-13 NMR chemical shifts of the PC carbonyl carbon at the surface layers indicated that PC polar headgroups are more separated and exposed to water molecules in emulsions than in vesicles. The N values were satisfactorily correlated with the chemical shift, that is, the degree of separation between the carbonyl groups at the surface. Although apoA-1 binding to the PC monolayers of emulsions brings about bending of the surface layers and creates local defects in the hydrocarbon regions in a similar manner as PC LW, the surface-core interaction seems to fill the defects with the core neutral lipids, compensates for the bending stress, and eventually increases the N value. Dependence of the core effect upon the acyl chain length of triglycerides implied important roles of the acyl chains in the surface-core interaction between PC and triglycerides.