The plasma level of LDL cholesterol (LDL-C) is an independent risk factor for atherosclerosis. Recently, homogenous assays were developed to measure the level of LDL-C directly. In some cases, however, these assays give inconsistent results probably due to their difference in measurement principles and reactivity. In this study, we therefore evaluated the specificity of three homogeneous assay reagents currently available commercially. In the experiment, measures of the homogeneous assays were compared with the cholesterol levels in the fractions separated by ultracentrifugation. We found that the reactivity to IDL and VLDL differed substantially among the assays studied.