A long-acting, pegylated variant of filgrastim (recombinant human granulocyte colony-stimulating factor (G-CSF)) with immunomodulating activity. Similar to endogenous G-CSF, this agent binds to and activates specific cell surface receptors, stimulating neutrophil progenitor proliferation and differentiation and selected neutrophil functions. The modification of this version of filgrastim with the addition of a Y-shaped branch of polyethylene glycol at a specific lysine residue (Lys 17) theoretically serves to extend the half-life of the agent.