Kinetics of SAPK activation by hematopoietic growth factors. After 1 hour in RPMI 1640 at 37°C, MC/9 cells were left untreated, or treated with saturating doses of GM-CSF (GM-CSF ), SLF (SLF ), or 0.2 mol/L NaCl (Na) as a positive control for the indicated times. Cell extracts were prepared and GST-c-Jun was used as both affinity reagent and substrate for an in vitro kinase assay. (A) The phosphorylation of c-Jun was assessed by autoradiography. (B) The fold activation of SAPK in cells treated with SLF (▪) or GM-CSF (○) compared to untreated cells was quantitated using densitometry.