(Upper panel) Stimulation of the proliferation of FDC-P1 cells by serial dilutions of 1 ng/mL GM-CSF or IL-3 and the selective inhibition of GM-CSF but not IL-3 by a monoclonal GM-CSF antibody. (Lower panel) The stimulating activity of postendotoxin sera for FDC-P1 cells is completely inhibited by the same GM-CSF antibody.