In this study, we investigated the neutrophil functions after chemotherapy and the effect of recombinant human granulocyte colony-stimulating factor (rhG-CSF) on neutrophil functions in 15 patients with pulmonary carcinoma. Neutrophil phagocytosis and superoxide production decreased 15 days after chemotherapy but not significantly. In 13 patients, the serum granulocyte colony-stimulating factor (G-CSF) concentration was determined before chemotherapy. Phagocytosis of the low G-CSF group (G-CSF < 30 pg/ml) decreased significantly after chemotherapy. On the other hand, phagocytosis of the high G-CSF group (G-CSF 30 pg/ml) showed no reduction. Superoxide production did not change significantly. These findings indicate that chemotherapy for pulmonary carcinoma decreases not only the absolute neutrophil count but also neutrophil functions, and that high concentration levels of serum G-CSF might ameliorate the decrease in neutrophil functions. Six patients received two consecutive courses of the same drugs, and after the second course of chemotherapy they also received 100 ig rhG-CSF on alternate days. During the course of rhG-CSF their neutrophil phagocytosis was enhanced even after chemotherapy. From these findings, it can be concluded that the treatment of rhG-CSF relieves the reduction of neutrophil functions after chemotherapy in patients with pulmonary carcinoma.