Fourier-transform infrared (FT-IR) spectroscopy was applied to examine human serum very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) in aqueous solution and in solid film for characterizing lipid components. On the basis of the FT-IR second-derivative spectra for standard samples of triglycerides, cholesterol esters and phospholipids, it was found that the band at 1746 cm - 1 for VLDL and the band at 1738 cm - 1 for LDL were mainly due to the unsaturated triglycerides and unsaturated cholesterol esters, respectively. The implications of ester C O stretching bands are discussed.