1. Any of several cytokines that are produced by a variety of cell types and that stimulate progenitor cells in the bone marrow to produce specific types of blood cells, especially white blood cells such as granulocytes. In vitro, these cytokines stimulate the formation of colonies of cells from a single cell.

2. Any of various recombinant forms of one of these cytokines, such as filgrastim, used as a drug.