Imatinib inhibits M-CSF– but not IL-3–stimulated growth of a c-fms–expressing cell line at therapeutic concentrations. FDC-cfms cells were stimulated with murine IL-3 (A) or rhM-CSF (C) and cell counts performed at 12 (♦), 24 (▪) and 48 () hours. The relationship between imatinib concentration and cell growth was predicted according to a sigmoidal model and used to calculate IC₅₀ values (B,D). Results are representative of 3 individual experiments. Error bars represent SEM.