The invention provides biologically active erythropoietin (EPO) 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-(X-Y)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.