The invention relates to novel modified erythropoietin (EPO) forms such as fusion proteins comprising a Fc portion of an Ig molecule and a target molecule having the biological activity of EPO. By selective altering of the amino acid sequences of the erythropoietin moiety as well as of the immunoglobulin moiety and the glycosylation pattern of erythropoietin fusion proteins with enhanced biological activity can be obtained. The invention relates also to novel nun-fused EPO molecules which have a pattern of cysteines or disulfide bonding which is distinct from human or animal EPO.