Abstract: A lipoprotein diluent useful in the preparation of a standard or reference material for assay procedures which comprises a stabilized aqueous solution of turbidity-potential lipoproteins. The solution contains a preservative including a polyhydroxy cryoprotective agent and/or an antibiotic, and may be used directly as a cholesterol or triglyceride standard or control. The solution may also include other stable components but is free from lipoprotein-degrading enzymes. In accordance with the method of the present invention, a standard or reference material is prepared by adding a diluent containing turbidity-potential lipoproteins to a lyophilized serum base, which contains enzymes.