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Fig. 8. Role of growth factors on the differentiating activity of pharmacological differentiating agents. U937 cells were incubated for 120 h in the absence (Control) or presence of hydroxyurea (100 µg/ml) or phenylbutyrate (1.5 mM) along with neutralizing antihuman GM-CSF (0.1 mg/ml) and antihuman IL-3 (0.15 mg/ml) or nonbinding isotypic control antibodies and assessed for CD11b expression (A) and clonogenic growth (B). All results represent the means of three separate experiments; bars, SE. Ps compare agents treated with blocking antibodies or control antibodies by the paired Students t test.