Complexes of a protein selected from the group consisting of VEGF-B167, VEGF-C, VEGF-D and processed VEGF-B186 and analogs thereof and the neuropilin-1 (NP-1) receptor, the extracellular domain or a ligand-binding fragment or analogue thereof; the use of such complexes in assays for growth factor proteins having substantially the same binding affinity for a cell surface receptor as VEGF-B167, VEGF-C, VEGF-D or processed VEGF-B186 and/or in promoting or antagonizing a cellular response mediated by VEGF-B167, VEGF-C, VEGF-D and/or processed VEGF-B186; and specific binding partners, e.g. antibodies, for such complexes.