We evaluated the performance of a commercially available cellulose acetate electrophoretic method for quantitating high-density lipoprotein cholesterol (I) in serum by comparing it to a method involving precipitation with dextran sulfate-500/Mg2+. In both methods, enzymic reagents are used for cholesterol measurement. For electrophoretic measurement of I the mean intramembrane CV ws 4.1% (at 220 to 360 mg/L) and the intermembrane CV ranged from 12.2 to 21.0% (at 220 to 880 mg/L). Interassay precision was significantly better for the precipitation method (CV = 3.9% at 390 mg/L). The electrophoretic procedure demonstrated significant measurement bias, both at high and low I concentrations. However, low-density lipoprotein cholesterol, measured electrophoretically correlated well with its calculated concentrations obtained by the precipitation method. Measurement of I by this electrophoretic procedure did not achieve the accuracy and reproducibility that have been demonstrated for precipitation methods and that are necessary for reliable clinical interpretation of results for I.