Differential scanning calorimetry (DSC) and x-ray diffraction studies were performed on chylomicrons and very low density lipoproteins (VLDL), from nonhuman primates fed saturated fat, isolated from lymph at two different temperatures (15 and 39 degrees C). When heated from -10 leads to 60 degrees C, chylomicrons and VLDL isolated at 15 degrees C had two endothermic transitions resulting from the melting of triglycerides (TG). Cooling resulted in the onset of crystallization of the TG core at 16-19 degrees C; 50% of the TG in the particles remained fluid at 4 degrees C. The initial DSC pattern was reproduced not on immediate reheating, but by storing samples at 4 degrees C overnight. Chylomicrons isolated at 39 degrees C contained a metastable (undercooled) TG core until cooled to the onset of TG crystallization at 16-19 degrees C. Reheating the particles with crystalline TG to body temperature (39 degrees C) resulted in a partially crystalline TG core, not an undercooled liquid. To ascertain the effect of particle structure on the TG physical properties, intact particles were compared with heat denatured particles and extracted lipids. The onset of crystallization was lower and the time necessary for isothermal crystallization was much greater for intact versus denatured particles or lipids. Only minor differences in physical properties as a function of particle size were found. It was concluded that: 1) the physical state of chylomicrons and VLDL TG core can be modified by isolation temperature; 2) particle structure affects crystallization but not melting of chylomicrons and VLDL TG; and 3) chylomicrons particle size does not markedly influence the physical properties of the TG core.