We reported previously that granulocyte colony-stimulating factor (G-CSF) can promote the invasion of human lung cancer cell lines in vitro. However, the exact mechanism of its stimulatory effect on invasion remains to be elucidated. In the present study we mainly focused our attention on the components of the plasminogen activation system in human lung cancer cell lines, because of the central role that plasminogen activators play in regulating extracellular proteolysis. We showed that G-CSF induced a dose-dependent increase in the urokinase-type plasminogen activator (uPA) activity in the conditioned medium of a PC-9 lung cancer cell line. When the amounts of uPA activity were quantitated by densitometry, we found that even at a concentration of 0.01 microg/ml, G-CSF had a stimulatory effect on the uPA release, while high concentrations caused a 3.6-fold increase at a maximum concentration of 1 microg/ml. A Western blot analysis of the conditioned medium confirmed the findings observed in a zymographic analysis. The observed increase in uPA protein was paralleled by a significant increase in the uPA mRNA levels after treatment with G-CSF. However, our experiments failed to identify any alteration in the plasminogen activator inhibitor (PAI) secretion caused by G-CSF. In addition, we also found the expression of G-CSF receptor by PC-9 cells, suggesting the possible pathway activated by G-CSF.