Recombinant human erythropoietin labeled covalently with biotin at sialic acid moieties has been prepared, and has been shown to possess high biological activity plus utility as a receptor ligand. Initially, the effects on biological activity of covalently attaching biotin to erythropoietin alternatively at carboxylate, amino, or sialic acid groups were compared. Biotinylation of erythropoietin...