The combination effect of subinhibitory and inhibitory concentrations of ceftazidime, granulocyte colony-stimulating factor (G-CSF) and polymorphonuclear leukocytes (PMNL) against Escherichia coli was investigated. PMNL obtained from healthy volunteers were incubated with different concentrations of G-CSF and ceftazidime for 180 min. The addition of 0.25 x MIC of ceftazidime or 1 x MIC significantly enhanced the bactericidal activity of PMNL. G-CSF at a concentration of 6,000 units/ml led to only a slight improvement in the bactericidal activity of PMNL. The combination of 6,000 units/ml G-CSF and ceftazidime in inhibitory as well as subinhibitory concentrations, however, showed a significant synergistic effect (p < 0.01) on the antibacterial activity of PMNL during the entire incubation period. Combinations of G-CSF and antibiotics could therefore be beneficial for infected patients, especially those with impaired cellular host defence.