Disclosed are a method and a reagent for quantifying HDL3 in a test sample without requiring laborious operations. The method for quantifying cholesterol in high-density lipoprotein 3 comprises reacting a test sample with one or more surfactants which react specifically with high-density lipoprotein 3, and quantifying cholesterol. When one surfactant is used, the surfactant is one selected from the group consisting of polyoxyethylene polycyclic phenyl ethers having an HLB of 12.5 to 15. When two or more surfactants are used, at least one of the surfactants is at least one selected from the group consisting of polyoxyethylene polycyclic phenyl ethers, and the two or more surfactants are combined so as to provide the total HLB of 12.5 to 15 of the combined surfactants.