Cholesteryl ester transfer to low density lipoproteins (LDL) was achieved either by incubation of a cholesteryl ester-rich lipid microemulsion with whole plasma or by incubation of the cholesteryl ester-rich microemulsion with lipoprotein-free plasma and LDL. Sequential ultracentrifugal isolation gave LDL that contained between 5 and 37% of the cholesteryl ester derived from the artificial cholesteryl ester donor. The labeling was essentially complete within 2 h of incubation. Resonance energy transfer of LDL, labeled with fluorescent cholesteryl esters and with the surface probe 5-(N-hexadecanoylamino) fluorescein, indicated that the added cholesteryl ester was located in the inner core of the lipoprotein. In addition, biological screening of the labeled LDL in cultured cells in vitro and in the rat in vivo showed that this labeling procedure had no detectable deleterious effect on LDL. This method of labeling lipoproteins allows a rapid and specific introduction of cholesteryl esters into lipoproteins.