Functional activity of peripheral blood neutrophils was assessed in eight patients at 4, 6, 8, 10 and 12 weeks following autologous bone marrow transplantation (ABMT). Functions studied included superoxide generation (O2-) intracellular killing of Staphylococcus aureus, phagocytosis and killing of Candida albicans. Neutrophils were tested following in vitro preincubation with 300 pM granulocyte-macrophage colony-stimulating factor (GM-CSF), 1.2 nM granulocyte colony-stimulating factor (G-CSF) or buffered solution (diluent) as control. Our data indicate that during the early period (weeks 4-6) following ABMT most of the patients exhibited diminished neutrophil oxidative metabolism, defective phagocytosis and killing of C. albicans and reduced capacity to kill S. aureus. In some patients a gradual increase in the functional activity of neutrophils occurred with time. Both GM-CSF and G-CSF induced in vitro amplification of (a) O2- production in response to fmet-leu-phe (FMLP) (b) phagocytosis and killing of C. albicans and (c) killing of S. aureus. This study suggests that GM-CSF and G-CSF may enhance the depressed functional activity of neutrophils following ABMT.