Background: The changes in vivo in immunocyte functions during chemotherapy that is administered in combination with granulocyte colony-stimulating factor (G-CSF) in humans have not been fully investigated. This study was designed to examine neutrophil functions and the activities of natural killer (NK) cells, during the administration of chemotherapy and G-CSF for the treatment of testicular cancer. Methods: Seven patients with germ cell tumors at stage IIA, IIB or IIIB, who were treated with bleomycin, etoposide and cisplatin (BEP), were enrolled in the study. Numbers and activities of neutrophils and NK cells were measured at various times during and after the first course of chemotherapy. Neutrophil phagocytosis was quantitated by flow cytometry with fluorescent latex beads. Bactericidal activity was measured in terms of colony-forming units. The activity of NK cells was measured by monitoring the release of 51 Cr. Results: After BEP chemotherapy, CD16 + and CD56 + cell counts, and neutrophil granulocyte counts decreased while there were no significant changes in the number of lymphocytes. Phagocytosis by neutrophils was enhanced after administration of G-CSF. The activity of NK cells was severely impaired after chemotherapy and did not change after administration of G-CSF.