Various partially or fully desialylated human erythropoietins were obtained by neuraminidase digestion of the hormone, without non-specific proteolysis and degradation of carbohydrates. Asialoerythropoietin showed a specific activity of 220-IU/mg protein in vivo, although that of the intact erythropoietin was 2.2 × 105 IU/mg. A linear relationship was found between the logarithm of the specific activity in vivo and the number of sialic acids. The asialoerythropoietin showed a four-times-higher specific activity in vitro compared with intact erythropoietin using mouse bone marrow cells. It also showed an approximately six-times-higher specific activity in a colony-forming assay for the erythroid colony-forming unit and the erythroid burst-forming unit. Partially or fully de-N-glycosylated erythropoietin derivatives also showed lower in vivo activity but higher in vitro activity than the intact erythropoietin, dependent on the number of sialic acids. To clarify the reason for the enhanced biological activity of asialoerythropoietin in vitro, the binding of intact 125I-erythropoietin or 125I-asialoerythropoietin to cells containing specific receptors for the hormone was analyzed. 125I-asialoerythropoietin bound to spleen cells from anemic mice approximately five times faster than did intact 125I-erythropoietin. The amount of 125I-asialoerythropoietin internalized by target cells, measured in the absence of NaN3, was four times higher than that of intact erythropoietin. These results demonstrate that asialoerythropoietin binds to its receptor faster than the intact form. This may be the main reason for the increased activity of asialoerythropoietin in vitro.