Human erythropoietin (EPO), which occurs only in minute quantities in human plasma, has been isolated from urine of patients and purified to homogeneity. A partial amino acid sequence from tryptic digests of the purified protein was selected for oligodeoxynucleotide probe synthesis. These probes were used to isolate the EPO gene from a human gene bank. The isolated gene was engineered into a eukaryotic expression vector and introduced into chinese hamster ovary cells commonly used for recombinant protein production. A specific amplification of the EPO gene copy number in these cells led to the secretion of milligram quantities of EPO per liter of cell supernatant. This protein has similar characteristics to native urinary EPO, regarding its electrophoretic mobility, glycosylation pattern and biologic activities in vitro and in vivo. A scaled-up industrial process provides recombinant EPO for its clinical use.