Provided is a method for quantifying soluble LR11 in a biological sample such as serum by an immunological means conveniently and accurately without the need of carrying out any complicated separation manipulation. An immunological quantification method for soluble LR11 in a sample derived from a mammal, including a step of treating the sample with at least one surfactant selected from a group consisting of a polyoxyalkylene alkyl ether, a polyoxyalkylene alkyl phenyl ether, an alkyl glycoside, an alkylthio glycoside, an acyl-N-methylglucamide and a salt of cholic acid.