Automated FRET imaging of membrane-bound receptor/ligand complexes can discriminate between a clustered organization of ligand/receptor complexes that occurs during the early endocytic stages following internalization and a random distribution characteristic of late stage disassociation of ligand from the receptor. In the case of the low density lipoprotein receptor (LDL-R) and its ligand, LDL, this feature of FRET imaging forms the basis of an assay to monitor the endosomal release of cholesterol into the cell and identify compounds which alter pH in the endosome thereby inhibiting the disassociation of ligand and cholesterol from the receptor, a mechanism that is involved in regulation of plasma/serum cholesterol.