Human urinary neuraminidase, an enzyme that releases sialic acid from hematopoietic factors found in urinary preparations, was partially characterized, and a method was developed to derive these hematopoietic factors free of enzyme activity. Neuraminidase in urinary preparations from healthy humans and aplastic anemic (AA) patients had optimal activity at pH 5.3 and hydrolyzed both α2 → 3 and α2 → 6 type ketosidic linkages of N-acetyl-neuramin lactose and α1-acid glycoprotein. When subjected to Sephacryl S-300 gel filtration, urinary neuraminidase showed a single peak of activity with an apparent molecular weight of 380,000 daltons, even under denaturing conditions (6 M guanidine hydrochloride). Furthermore, among a variety of compounds tested, no potent inhibitor of the enzyme was found. Heat treatment of AA urinary preparations eliminated about 80% of neuraminidase activity, while successive two-step ethanol precipitation eliminated residual enzyme. Erythropoietin, megakaryocyte colony-stimulating factor (CSF) and granulocyte/macrophage CSF activities were retained after these treatments.