The carbohydrate composition of human erythropoietin (epo) was determined by micro-GLC. Enzymic removal of most of the sugars results in aggregation of glycosidase-treated epo, loss of biological activity when assayed in mice, and retention of activity when assayed in marrow cell cultures or by RIA. Endoglycosidase F causes the removal of most of the carbohydrates indicating that the oligosaccharides are asparagine linked. The lack of O-linked sugar is confirmed by the absence of N-acetylgalactosamine. These findings indicate that the oligosaccharide portion of epo, although required for action in vivo, is not required for interaction with the target cells of the blood-forming system.