Granulocyte colony-stimulating factor (G-CSF) is a major regulator of the development of antibacterial neutrophilic granulocytic leukocytes (neutrophils). In keeping with the functions of both G-CSF and neutrophils, the murine molecule was first purified from medium that had bathed the dissected lungs of animals previously treated with a bacterial cell-wall extract (1). Several years later, the equivalent human molecule was reported in the supernatent from a carcinoma cell line that inadvertently expressed G-CSF in extraordinarily high concentrations (2,3). The human material was found to be a glycoprotein of approx 19 kDa, which was variably acidic depending on the proportion of sialic acid in the carbohydrate component (3). It was later found that the carbohydrate was optional for biologic activity and that the core protein comprised 174 amino acids (4,5) (Table 1).