To provide a method for assaying soluble LR11 in a biological sample, which method realizes a simple and accurate assay of soluble LR11 present in the sample by immunological means without requiring isolation of soluble LR11 from the biological sample (e.g., a serum sample).The method of the invention for immunologically assaying soluble LR11 present in a biological sample, characterized in that the method includes treating the sample with at least one surfactant selected from among one or more sulfobetaine amphoteric surfactants and one or more amidosulfobetaine amphoteric surfactants.