SD/01, a sustained-duration molecule, has been developed by adding a poly [ethylene glycol] molecule to the filgrastim molecule. The pegylation does not change the properties of filgrastim, except that the plasma clearance is decreased and plasma half-life is increased. Increasing the duration of the biological effects of filgrastim may offer certain groups of patients better benefits. Early clinical studies have been encouraging with no serious toxicities noted.