A method for quantitatively determining LDL cholesterol, including the steps of adding to serum a surfactant selected from among polyoxyethylenealkylene phenyl ethers and polyoxyethylenealkylene tribenzylphenyl ethers and a cholesterol-assaying enzyme reagent so as to preferentially react cholestrols in high density- and very low density-cholesterols among lipoproteins, and subsequently determining the amount of cholesterol that reacts thereafter. This method can eliminate the necessity for pretreatments such as centrifugation and electrophoresis, enables the quantitative determination to be conducted in an efficient, simple manner, and can be applied to various automatic analyzers.