c-Fms signaling primes macrophage response to lipopolysaccharide (LPS) or immune complex stimulation. Fully differentiated, bone marrow-derived macrophages pretreated with macrophage colony-stimulating factor (M-CSF) for 3 hours in the absence or presence of 5 μM GW2580 or imatinib followed by stimulation with (a) low-dose LPS (1 ng/mL) or (b) FcRgII/III cross-linking (20 μg/mL plate-bound 2.4G2 antibody). After 24 hours of culture, tumor necrosis factor (TNF) in the supernatants was measured by enzyme-linked immunosorbent assay. Values are the mean ± standard error of the mean. **P < 0.01 compared with unprimed stimulated cells without inhibitor. Results are representative of at least three independent experiments. IC, immune complex.