PCT No. PCT/US95/01696 Sec. 371 Date Jan. 21, 1997 Sec. 102(e) Date Jan. 21, 1997 PCT Filed Feb. 8, 1995 PCT Pub. No. WO95/21864 PCT Pub. Date Aug. 17, 1995The invention relates to a composition comprising antibodies or binding portions thereof which bind to a naturally occurring or recombinant G-CSF receptor extracellular domain, and reduce tyrosine phosphorylation of a JAK molecule. The invention also relates to methods of using this composition for inhibiting or decreasing tyrosine phosphorylation of a JAK kinase and for inhibiting or decreasing G-CSF interaction with its receptor. Also described are antibodies which bind to a G-CSF receptor extracellular domain and which are produced by immunizing an animal with cells transfected with DNA encoding a G-CSF receptor.