Erythropoietin (Epo), a glycoprotein produced primarily by the kidney, is the principal factor that regulates erythropoiesis by stimulating the proliferation and differentiation of erythroid progenitor cells. The biological effects of Epo are mediated by the erythropoietin receptor (Epo R). The presence of a soluble form of the Epo R has also been detected on human sera. Recombinant soluble Epo R binds Epo with high affinity and is a potent Epo antagonist.