Monocyte chemoattractant protein-1 (MCP-1), which is required for full development of glucan-induced granulomas in the rat, is expressed in the walls of blood vessels at sites of glucan embolization. Early (1 hour) vessel wall expression of MCP-1 is temporally and anatomically linked to the transient accumulation of neutrophils, even though these cells are not present within definitive lesions. To ascertain the potential pathophysiologic role of neutrophils in glucan-induced granuloma formation, rats were neutrophil-depleted using specific antiserum. There was a marked reduction in mean granuloma size and number in neutrophil-depleted animals when compared with neutrophil-sufficient controls. To determine potential mechanisms through which neutrophils may participate in granuloma formation, the antioxidant enzymes superoxide dismutase and catalase were administered to neutrophil-sufficient animals that had received glucan. Superoxide dismutase treatment did not reduce granuloma formation, whereas catalase treatment resulted in decreased granuloma size, suggesting that H2O2plays an important role in this process. The local expression of MCP-1 mRNA and protein, as determined by in situ hybridization and immunohistochemical analysis, respectively, was decreased in both neutrophil-depleted and catalase-treated animals but not in superoxide dismutase-treated rats. Quiescent human umbilical vein endothelial cells incubated with either H2O2or activated neutrophils secreted MCP-1. These data indicate that neutrophils and H2O2are required for both full granuloma development and early blood vessel wall-associated MCP-1 expression after glucan infusion. These in vivo data, coupled with in vitro data that indicate that both catalase-sensitive reagent H2O2and neutrophil-derived reactive oxygen intermediates (ie, H2O2) can induce MCP-1 secretion by human umbilical vein endothelial cells, support the hypothesis that neutrophils and neutrophil-derived products (H2O2) influence granuloma formation through induction of local MCP-1 expression.