Interferon-gamma (IFN-gamma) has potent antiproliferative effects on the endothelium, although the specific mechanisms responsible for this effect are not clear. We tested the hypothesis that suppression of endothelial cell proliferation by IFN-gamma is mediated by an increase in xanthine oxidase-derived O2-.. Human umbilical vein endothelial cells (HUVEC) were exposed to recombinant human IFN-gamma. We found that [3H]thymidine uptake decreased (p < 0.05) with increasing doses of IFN-gamma. Treatment of HUVEC with the xanthine oxidase inhibitor allopurinol or the O2-. scavenger superoxide dismutase had no effect (p > 0.05) on [3H]thymidine uptake of IFN-gamma-treated cells. In parallel, IFN-gamma decreased (p < 0.05) HUVEC cell counts, while allopurinol again had no effect (p > 0.05) on cell counts of IFN-gamma-treated or control HUVEC. In addition, xanthine oxidase activity of HUVEC did not (p > 0.05) increase following treatment with IFN-gamma. We conclude that IFN-gamma suppresses HUVEC proliferation by a mechanism independent of O2-. production by xanthine oxidase.