Severe congenital neutropenia (SCN) is a disorder of myelopoiesis characterized 
by severe neutropenia or absence of blood neutrophils secondary to a 
maturational arrest at the level of promyelocytes. We examined peripheral blood 
mononuclear cells (PBMC) of SCN patients who demonstrated normalization of their 
blood neutrophil counts in a phase II clinical study with recombinant human 
granulocyte colony-stimulating factor (rhG-CSF). When stimulated in vitro with 
bacterial lipopolysaccharides (LPS), PBMC of those SCN patients produced G-CSF 
activity, as judged by proliferation induction of the murine leukemia cell line, 
NFS-60. Western and Northern blot analysis showed G-CSF protein and G-CSF-mRNA 
indistinguishable in size from those of normal controls. We conclude that PBMC 
of the SCN patients tested are capable of synthesizing and secreting 
biologically active G-CSF in vitro.