Neutrophils from 13 children who received G-CSF for the collection of peripheral blood progenitors while they were in haematological steady state were studied at various times after G-CSF injection for Fc gammaR expression (Fc gammaRI or CD64, Fc gammaRII or CD32, and Fc gammaRIII or CD16) and for their ability to exert antibody-dependent cell cytotoxicity (ADCC) through Fc gammaRI. Changes in IFNgamma, IL8, IL10, MCP1 and TNF alpha mRNA levels in peripheral blood cells were also studied 4 h and 24 h after the first G-CSF injection. Fc gammaRI expression increased strongly after 24 h and then remained at the same level throughout treatment. In contrast, Fc gammaRIII expression sharply decreased at day 1 and diminished even further thereafter. No change in Fc gammaRII was observed. ADCC exerted by neutrophils through Fc gammaRI started to increase after 24 h with the peak level at day 5. Cytokine mRNA analyses indicated a reproducible and strong increase of IL8 mRNA (11/13 children) after 24 h, whereas the changes in the mRNA levels of the other cytokines tested were more heterogenous (TFNgamma: three; IL10: six; MCP1: five: TNF alpha: four, of the 13 children). Therefore this study opens the way to an optimized therapeutic schedule for the combined use of G-CSF and monoclonal antibodies in adjuvant immuno-intervention.