Granulocyte-colony stimulating factor (G-CSF) is one of the glycoproteins called colony-stimulating factors (CSFs). It has been shown that the target of the actions of CSFs are not limited to hematopoietic cells but can also affect the proliferation of nonhematopoietic cells. Some clinical investigations have shown the presence of cell surface receptors for G-CSF in lung cancer cells and autologous production of G-CSF in various human cell lines derived from non-small-cell lung cancer (NSCLC). The purpose of this investigation was to compare serum levels of G-CSF in NSCLC patients to a control group, to assess pre- and post treatment levels of G-CSF in relation to levels of commonly accepted tumour markers such as carcinoembryonic antigen (CEA) and cytokeratin fragment 19 (CYFRA 21-1), and to define the sensitivity of G-CSF in NSCLC. In this study, the serum levels of tumour markers were measured in 34 patients with NSCLC and in 20 healthy subjects. Serum samples were drawn before surgery and 10, 30, 90, 180 and 270 days after surgery. G-CSF and CEA were assayed using ELISA system and CYFRA 21-1 was measured by radioimmunoassay (RIA). Preoperative level of G-CSF was significantly increased in cancer patients relative to the control group. Concentrations of G-CSF and CYFRA 21-1 were decreased on the 10th day, but CEA on the 30th day after operation. The diagnostic sensitivity of G-CSF was 66%, CEA--62% and CYFRA 21-1--51%. Combined use of two markers increased the sensitivity in comparison to the use of G-CSF only. These results suggest that G-CSF may be useful in diagnostic and monitoring of NSCLC, but they need further studies.