Internalization kinetics of wt, [S749A], and [S749D]G-CSF-R in 32D.cl8.6 cells. 32D cells expressing wt (squares), [S749A] (triangles), or [S749D]G-CSF-R (circles) were allowed to bind biotinylated anti–G-CSF-R antibodies in the absence (open symbols) or presence (closed symbols) of G-CSF for one hour at 4°C and were subsequently incubated at 37°C for 0, 5, 15, or 30 minutes at 37°C before staining with SA-PE to determine surface-bound G-CSF-R. The percentage of internalized G-CSF receptors was determined by assessing peak channel values of fluorescence at the different time points. This is a representative experiment out of 3 independent experiments.