The cDNA for human erythropoietin (hEPO) inserted into the mammalian expression vector BMCGSNeo was introduced into SP2/0-Ag14 cells and a transformant producing large amounts of hEPO was established. The recombinant hEPO in conditioned medium was purified by immunoaffinity and gel filtration column chromatographies. The purified hEPO had full in vitro biological activity, but low in vivo biological activity.