Abstract: Chimeric proteins, which bind a cell surface receptors, useful in transporting a selected substance present in extracellular fluids, such as blood or lymph, into cells; quantitative assays for the selected substance using chimeric proteins; DNA encoding the chimeric proteins; plasmids which contain DNA encoding the chimeric proteins; mammalian cells, modified to contain DNA encoding the chimeric proteins, which express and, optionally, secrete the chimeric proteins; a method of producing the chimeric proteins; a method of isolating the chimeric proteins; a method of using the chimeric proteins to assay the selected substance; and a method of reducing extracellular levels of the selected substance through administration of the chimeric protein, which results in transport of the selected substance into cells.