A compound is disclosed herein which is water soluble and is useful in assay procedures for measuring serum cholesterol. In one embodiment, the compound comprises a cholesterol ester having the general formula R.sub.1 --R.sub.2 --[Cholesterol Base], in which R.sub.1 is a water soluble, nonionic surfactant and R.sub.2 is a dicarboxyl group bonded by ester linkages to R.sub.1 and to the cholesterol. In an alternate embodiment the compound comprises a pregnenolone derivative having the general formula R.sub.3 --[Pregnenolone Base]--R.sub.4. A compound in accordance with the present invention is useful by itself or in combination with blood serum as a standard or reference material for cholesterol assay procedues.