The gastrointestinal epithelium is continuously exposed to reactive oxygen metabolites that are generated within the lumen. In spite of this exposure, the healthy epithelium appears unaffected, suggesting efficient mechanisms for protection against these potentially cytotoxic oxidants. The objective of this study is to characterize the interaction between purified gastric mucin and hydroxyl radicals generated from the interaction between ferric iron and ascorbic acid. We found that both native and pronase-treated mucin effectively scavenged hydroxyl radical and that the scavenging properties were not significantly different. The effective concentration of mucin required for a 50% reduction in malondialdehyde production was approximately 10 mg/ml for both native and pronase-treated mucin. In addition, the iron-ascorbic system produced a dramatic decrease (greater than 50%) in the specific viscosity of mucin that was inhibited by catalase, deferoxamine, and mannitol. Superoxide dismutase had no effect. These data suggest that hydroxyl radicals derived from the iron-catalyzed decomposition of hydrogen peroxide are responsible for the depolymerization of native mucin. We propose that mucin may provide protection to the surface epithelium of the gastrointestinal tract by scavenging oxidants produced within the lumen; however, it does so at the expense of its viscoelastic properties.