Fluoride ion (F-) is an effective activator of the respiratory burst in neutrophils, as indicated by its ability to induce O2- production by these cells. Other halide ions did not activate the burst, Cl-, in particular appeared to antagonize the effect of F- on O2- production. F- stimulated O2- production showed a requirement for Ca++, but was independent of other exogenous cations. Neither phagocytosis nor degranulation were necessary for respiratory burst activation by F-. The effect of F- on the respiratory burst was reversible. Washing the cells after treatment with F-, while they were still producing large amounts of O2-, returned them to the resting state. They could then be stimulated again to produce O2- in amounts equivalent to those originally produced. Our experiments indicated that restimulation did not represent the activation of a population of cells that had not been activated during the initial exposure to F-, nor did it represent serial activation of different subpopulation of the O2- forming enzyme molecules present in a given cell. Rather, our data suggest that the entire population of O2- forming enzyme molecules was activated in a reversible fashion by F-.