Imatinib inhibits monocyte and/or macrophage colony formation. (A) Normal BMMNCs were stimulated with 4HGF (IL-3, IL-6, G-CSF, GM-CSF) (□), 4HGF plus α-c-kit antibodies (▦), 5HGF (IL-3, IL-6, G-CSF, GM-CSF, SCF) (▪), or 5HGF plus α–c-kit antibodies (▨), and monocyte and/or macrophage colony growth examined. Growth of monocyte and/or macrophage (▪) and eosinophil (□) colonies from CD34⁺ progenitors stimulated with M-CSF (B) or GM-CSF (C) was also examined. The 4HGF (A) or 0 μM imatinib controls (B,C) were used to set the value for maximal growth, and all other data points were normalized to this value. ELISA was used to determine whether GM-CSF–stimulated monocyte cultures produced autocrine M-CSF (D) (♦, 0 μM imatinib; ▪, 1.0 μM imatinib, ▴, 5.0 μM imatinib). Results are representative of 3 individual experiments. *P < .05. Error bars represent the standard error of the mean (SEM).