We have developed a new analysis method for lipoproteins in serum by high-performance liquid chromatography using a sulfopropyl-ligand column with eluents containing magnesium nitrate. The magnesium ion anchors lipoproteins to the ligands on the column gel. Lipoproteins are eluted from the column with a magnesium nitrate concentration gradient and detected by postcolumn reaction using a reagent containing cholesterol esterase and cholesterol oxidase. High-density lipoprotein, low-density lipoprotein, and very-low-density lipoprotein were eluted in order from the column. The within-assay and between-assay coefficients of variation for cholesterol concentration in lipoproteins were 1.1-3.7 and 1.3-5.8%, respectively. The correlation coefficients between the values of total cholesterol, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol obtained by the new method and those obtained by an enzymatic method using an automated chemical analyzer were 0.940, 0.979, and 0.909, respectively. The new method was successfully applied to the analysis of plasma lipoproteins of patients with hyperlipidemia.