A polyethylene glycol-modified erythropoietin (PEG-modified EPO) obtained by chemically modifying the lysine residue at the 52-position of natural erythropoietin (natural EPO) with polyethylene glycol. To enhance the long-lasting drug effect of EPO without damaging the physiological activity of EPO which is a sugar chain-rich glycoprotein, it has been required to develop a PEG-modified EPO having an extremely high long-lasting drug effect by introducing PEG into a controlled binding site at a controlled number of binding molecules. The above-described PEG-modified EPO shows a high long-lasting drug effect, thereby solving these problems.
