This invention provides methods of identifying ligands that are internalized into a cell. The methods typically involve i) contacting the cell with a reporter non-covalently coupled to a ligand; ii) dissociating the reporter from the ligand and removing dissociated reporter from the surface of the cell; and iii) detecting the reporter within said cell (if any is present) where the presence of the reporter within said cell indicates that the ligand binds to an internalizing receptor and is internalized.