Neutrophil function was studied by nitroblue tetrazotium (NBT) reduction, superoxide anion (O2·) production, chemotaxis, and bactericidal activity in 9 children with HBsAg-positive chronic active hepatitis (CAH). NBT reduction and O2· production were higher in resting neutrophils from the children with CAH than from the controls, but the production of O2· was depressed after stimulation with zymosan. No defect was observed in both random and direct locomotion, whereas a significant decrease was present in bactericidal activity. To evaluate the role of virus components, purified preparations of HBsAg and HBcAg were added to neutrophils from normal children. The incubation with such products inducedin vitrothe same modifications that were observed in the neutrophils from children with CAH. In addition, defects in phagocytosis and killing ofCandida albicansas well as in chemotaxis were demonstrated. The production of O2· was reduced in the neutrophils, stimulated by zymosan and previously opsonized with HBsAg-positive serum.