The role of oxygen metabolites in mediating virucidal activity was studied in two cloned macrophage-like cell lines. The parental cell line, J774.16, upon appropriate stimulation with either phorbol myristate acetate (PMA) or aggregated immunoglobulin, is induced to oxidize glucose via the hexose monophosphate shunt and produce O2- and H2O2. A variant derived from it, clone C3C, is defective in oxidative metabolism and cannot be stimulated to produce O2- or H2O2. Significant differences in yields of vesicular stomatitis virus (VSV) between stimulated clone 16 cells and unstimulated cells could be obtained only when low multiplicities were used for infection. Under the same conditions, PMA stimulation of the variant clone C3C produced no reduction in yields. The effect of PMA on virus yields in clone 16 was short-lived and dose dependent. PMA stimulation of either cell line had no effect on the number of infectious centers, suggesting that the antiviral effect was likely to be an extracellular, rather than an intracellular, one. Using glucose oxidase plus aglucose to generate H2O2 in solution, we observed that H2O2 alone is capable of killing limited amounts of VSV. The inactivation of VSV, both by H2O2 in solution and by activated clone 16 cells, could be inhibited by catalase. We conclude that intracellular resistance to VSV is primarily mediated through nonoxidative mechanisms, since activated macrophages can kill only a limited number of infectious virus particles extracellularly by means of secreted H2O2.