Competition binding experiment with HL60 cells. A total of 2 × 10⁶ cells mixed with different concentrations of human GM-CSF or DT388–GM-CSF and 0.2 μCi ¹²⁵I–labeled GM-CSF in 150 μL RPMI1640 with 2.5% BSA and 0.2% sodium azide and 20 mmol/L HEPES pH 7.2 and incubated for 1 hour at 37°C. Cells were centrifuged through a phthalate oil mixture and free and bound counts per minute determined. Scatchard analysis was used to determine kds.