Evidence from animal studies indicates that ozone (O3), the major component of environmental photochemical smog, depresses various intracellular hydrolytic enzymes and increases susceptibility to microbial infections. It would appear possible that O3 induced alterations in normal leukocyte functions might underlie some of these reported alterations in human response to infectious agents. A study was designed to assess the effect of O3 on peripheral blood leukocytes from 21 healthy young human males exposed to 784 micrograms/cu n of ozone for 4 hr. The capability of polymorphonuclear neutrophils to phagocytize and kill microorganisms of respirable size was evaluated and phagocytic and bactericidal rates were determined. A significant decrease in intracellular killing was seen at 72 hr postexposure (p < 0.001). A decrease in phagocytic ability was also noted at 72 hr (p < 0.05). These findings indicate that O3 in low concentrations has a transient effect on leukocyte functions in humans. Such an effect may be mediated by alterations in cell membrane, opsonization functions, and/or interference with intracellular enzyme synthesis.