Abstract The plasma enzyme lecithin-cholesterol acyltransferase was shown to be able to utilize lecithin-cholesterol mixed dispersions as substrate for esterification. However, dispersion with a lecithin/cholesterol molar ratio of at least 3 are required for effective esterification of dispersed cholesterol. Furthermore, dispersed cholesterol and lecithin appear to serve as substrate only after their incorporation into residual fraction ( d > 1.210) proteins to yield newly formed lipid-protein complexes, the majority of which have an apparent hydrated density between 1.063 and 1.210. This appears to reflect the ability of the esterifying enzyme to carry out effective transfer of the fatty acyl group from lecithin to cholesterol only when these components are associated with certain lipid-binding proteins in a specific structural arrangement. Both the molar ratio of lecithin/cholesterol and the ratio of these lipids to the combining proteins seem to influence the structural arrangement of the formed lipidprotein complexes. The proteins capable of associating with dispersed cholesterol and lecithin are almost absent in residual fractions isolated from fresh human plasma. Incubation of the plasma at 37° appears to promote the formation of these proteins and with it the utilization of lecithin-cholesterol dispersions as substrate for cholesterol esterification.