We explored ex vivo alterations in the cytokine release of stimulated blood cells taken from 8 patients with hematological malignancies who, after chemotherapy or radiotherapy developed leukopenia, and were treated for 3-7 days subcutaneously with granulocyte colony stimulating factor (G-CSF), daily, dose of 5 microg/kg of body weight. Blood was also taken from 8 healthy controls not treated with G-CSF and from patients before and 24 h after last dose of G-CSF and ex vivo treated with interferon (IFN) inducers: Newcastle disease virus (NDV), phytohemagglutinin (PHA), concanavalin A (Con A) and with tumor necrosis factor (TNF) inducer--lipopolysaccharide (LPS). Blood cells of patients before G-CSF treatment exhibited ex vivo a low ability to produce IFN-gamma in comparison to controls. After G-CSF therapy a significant increase in IFN-alpha production ability was detected. We conclude that G-CSF treatment for 3-7 days does not only increase the number of white blood cells (WBC) and neutrophilic granulocytes but also modify the host response of patients with hematological malignancies to microbial infections.