Lecithin–cholesterol acyltransferase (LCAT) is synthesized by the liver and secreted into the bloodstream, where it is believed to act principally on the surface of high-density lipoproteins (HDL). The first attempts to isolate LCAT yielded only partially purified preparations even though a variety of chromatographic techniques were used, including hydroxylapatite adsorption, salt precipitation, ultracentrifugation, and affinity chromatography. Lecithin–cholesterol acyltransferase has been one of the most difficult enzymes to characterize from the physical, chemical, and enzymological points of view. The painstakingly slow early development of an efficient purification procedure and the instability of the enzyme precluded rapid progress. For the assay of LCAT activity, a stable, efficient, and uniform substrate that can be added to the enzyme source under conditions such that the concentration of substrate is not rate limiting is required. This method permits the preparation of large amounts of stable, efficient homogeneous and well-defined substrate that is suitable for measuring the enzyme activity in plasma fractions.