Reactive oxygen species can directly damage tissue. In this setting, amplification of tissue damage also occurs through infiltration of inflammatory cells either acutely or chronically. Several recent studies suggest that reactive oxygen species stimulate production of certain chemokines, which are potent chemoattractants for inflammatory cells. In the present study, we examined whether oxidants, generated by the combination of xanthine and xanthine oxidase (X/XO), alter chemokine production by monocytes and U937 cells. Our findings demonstrate that X/XO stimulates monocytes, but not U937 cells, to produce increased amounts of interleukin-8 (IL-8) and monocyte chemoattractant protein. This effect is attenuated by pretreatment with dimethylsulfoxide (DMSO), a scavenger of hydroxyl radicals, but is not affected by superoxide dismutase or catalase. In contrast, X/XO-induced cytotoxicity, evidenced by lactate dehydrogenase release, is mediated primarily by hydrogen peroxide, as catalase reverses this effect. Finally, exposure to X/XO causes an increase in nuclear factor kappa B (NF-κB), and this effect is attenuated by DMSO. These studies suggest that reactive oxygen species can induce production of molecules that amplify inflammation through attraction of inflammatory cells. It appears the hydroxyl radical is the principal oxidant species: involved in stimulation of chemokine production.