Internalization of ligated G-CSF-R complexes. (A) 32D[WT] and 32D[mDA] cells were allowed to bind biotinylated–G-CSF at 4°C and were subsequently incubated at 37°C for various times before staining with SA-PE to determine surface-bound G-CSF. (Bold line) 0 minutes; (dotted line) 30 minutes; (thin line) 90 minutes; (shaded histogram) 0 minutes, with initial binding in the presence of excess nonbiotinylated–G-CSF. This is representative of four independent determinations. (B) Rate of internalization of¹²⁵I-G-CSF by 32D[WT] (▪) and 32D[mDA] (▵) cells, expressed as the percentage of total specific binding that was resistant to acid washing at each time point. Similar results were obtained in a duplicate experiment.