The effects of human recombinant granulocyte colony stimulating factor (G-CSF) on neutrophil recovery have been studied in patients undergoing bone marrow transplantation for haematological disease. Nine patients (five autografts and four allografts) were studied while receiving daily doses of G-CSF (range 2-20 micrograms/kg body weight) were compared to eight patients (four autografts and four allografts) who did not receive G-CSF as a control group. In both groups flow cytometry was used to determine neutrophil IgG Fc receptor (FcRII, FcRIII) expression. Phagocytosis and metabolic burst was assessed using IgG opsonized bacteria. The patients' neutrophils were studied prior to conditioning in autografts and donor cells were studied in allografts. Studies were repeated at neutrophil recovery (neutrophil count 1.0 x 10(9)/l) and ten days post recovery. At recovery FcRII expression was slightly increased in all groups, whereas the number of cells expressing FcRIII was reduced. This reduction was significant in the patients receiving rG-CSF at recovery and post recovery compared to the pretreatment levels. The ability to phagocytose bacteria in cells from patients receiving G-CSF was also reduced. The associated metabolic burst was significantly reduced in the autograft group but not the allograft group receiving rG-CSF. The reduced function and reduced expression of FCRIII suggests the presence of immature neutrophils in patients receiving rG-CSF post BMT.