1. The carbohydrate chain protects rhG-CSF from polymerization and/or conformational alterations associated with physicochemical changes, elevation of pH or temperature fluctuations. 2. The carbohydrate chain of rhG-CSF prevents loss of its biological activity in normal human serum by inhibiting proteinase activity. 3. These facts indicate that the carbohydrate chain of rhG-CSF has a markedly important role in maintaining the stability of the protein itself as well as in effecting the exertion of its biological activity.