the invention provides biologically active erythropoietin conjugate compositions wherein epo is covalently conjugated to a non - antigenic hydrophilic polymer covalently linked to an organic molecule that increases the circulating serum half - life of the composition . the invention thus relates to epo derivatives described by the formula epo — n where epo is erythropoietin or its pharmaceutical acceptable derivatives having biological properties of causing bone marrow cells to increase production of reticulocytes and red blood cells , x is peg or other water soluble polymers , y is an organic molecule that increases the circulating half - life of the construct more than the peg alone and n is an integer from 1 to 15 . other molecules may be included between epo and x and between x and y to provide the proper functionality for coupling or valency .