Lipid-lipid interactions between the core and monolayer have been studied by using reconstituted high-density lipoproteins (rHDLs) composed of apoHDL3 with either dipalmitoylphosphatidylcholine (DPPC) or egg phosphatidylcholine (egg PC) as the monolayer and either cholesteryl oleate (CO) or triolein (TO) as the core. The effect of the monolayer on the core was observed by deuterium nuclear magnetic resonance (2H NMR) studies of rHDLs containing the core component cholesteryl [18, 18, 18-2H3] oleate [( 2H3]CO) or tri[16,16-2H2]oleoylglycerol [( 2H6]TO) surrounded by a monolayer of either DPPC or egg PC as a function of temperature. The reverse effect, that of the core on the monolayer, was examined by both 2H and 31P NMR studies of rHDLs containing [5,5-2H2]PC in the presence of CO or TO as a function of temperature. The 2H NMR line widths of [2H3]CO and [2H6]TO were considerably broader and showed a greater temperature dependence in rHDLs containing DPPC than in those containing egg PC. Similarly, the C-2H order parameters of [2H2]PC were higher and showed a greater temperature dependence in rHDLs containing CO than in those containing TO. In contrast, the 31P NMR line widths were identical for both [2H2]-PC/CO/apoHDL3 and [2H2]PC/TO/apoHDL3 at 25 and 6 degrees C, showing only a slight temperature dependence. Thus, acyl chains of both the monolayer and core components show increased order when in contact with neighboring lipids of higher order. The data demonstrate a direct effect of core cholesteryl esters and triglycerides with the phospholipid monolayer of HDL.