Phosphorylation of ERK1, ERK2, and p38 MAPK in human neutrophils stimulated by GM-CSF. Cells were stimulated with indicated concentrations of GM-CSF for 10 minutes at 37°C or stimulated with 5 ng/mL GM-CSF for indicated periods at 37°C. Phosphorylation of ERK1, ERK2, (upper 2 panels) and p38 MAPK (lower 2 panels) was analyzed by immunoblotting using antibody against phosphorylated form of each protein. The cell lysates equivalent to 1.9 × 10⁶ cells were loaded onto each lane. The results shown are representative of three independent experiments.