Proliferation of PU.1 deficient cells was reduced in IL-3 and absent in G-CSF and GM-CSF as compared with control cells. Mononuclear cells isolated from neonates were incubated in IL-3 (1% SN), G-CSF (10 ng/mL), or GM-CSF (10 ng/mL) for 4 days. Proliferation was measured by colorimetric assessment of MTT reduction and by counting viable cells at the end of the culture period. Results for cellular proliferation are presented as the mean ± SD of absorbance. Similar results were obtained for spleen and bone marrow cells (not shown). Note an approximately threefold reduced proliferation of PU.1 deficient cells (▪) in IL-3 compared with control (▨) and no proliferation in G-CSF or GM-CSF detectable above the baseline (medium only conditions).