Proliferative response of BM-derived macrophages to GM-CSF. BM-derived macrophages pooled from multiple inbred WT(+/+) and null (−/−) mice (5 × 10⁴/well) were incubated with the indicated concentrations of GM-CSF for 5 days, and the proliferative response was assayed by the MTT assay as described in Materials and Methods. Results are presented as the mean ± the standard deviation of triplicate determinations. This experiment is representative of five independent experiments. Asterisks denote a significant difference (P < .05) in proliferation between cells from WT and null mice at that dose (Student's t-test).