Protection of pigs towards PRRSV infection mainly depends on the presence of antibodies in the pig that block virus infection of macrophages. Those neutralizing antibodies are directed against the part of the virus that mediates attachment to the receptor sialoadhesin, which is used by the virus to enter the primary target cell, the macrophage. We have identified a viral protein that binds to sialoadhesin. Identification of the GP5 protein as the putative PRRSV ligand for sialoadhesin clearly opens perspectives for subunit vaccine development.