The human receptors for poliovirus (hPVR) are members of the immunoglobulin superfamily. Whereas the two membrane-bound isoforms, hPVRα and hPVRδ, share identical three-domain extracellular portions, their C- terminal cytoplasmic parts differ considerably. This feature is well conserved in the corresponding monkey proteins AGMα1, AGMδ1, and AGMα2. The cellular function of these proteins is presently unknown. In this short communication we report that hPVRα and possibly also AGMα1 and AGMα2, but not the δ isoforms, are phosphoproteins. The phosphorylation occurs at a serine in the cytoplasmic tails of these receptors. We further present evidence suggesting that the kinase responsible for the phosphorylation is calcium/calmodulin kinase II.