The cholesterol derivative, represented by the following formula (1): ##STR00001## wherein R represents a C3 C23 saturated or unsaturated aliphatic hydrocarbon residue having an optional substituent, and, to the cholesterol backbone, --OH, --CHO, --COOH, --OOH, or an epoxy group may be added, specifically binds to .beta..sub.2-glycoprotein. By use of the derivative, .beta..sub.2-glycoprotein or a similar substance can be assayed in a very practical manner. Through the assay, a disease can be detected in a very practical manner.