Hydrolyzed carrageenan is used to induce ileocecal inflammation in laboratory animals. We used ileal epithelial cell monolayer cultures (IEC18) to study the cellular and paracellular injurious effects of hydrolyzed carrageenan via an examination of its effects on deoxyribonucleic acid synthesis, chromium release, and cell morphology. Phase-contrast microscopy showed that carrageenan-treated cells initially contracted and pulled away from neighboring cells. Cell and viability counts illustrated that hydrolyzed carrageenan retarded cell growth and eventually caused cell death. [3H]Thymidine incorporation revealed that hydrolyzed carrageenan at a concentration of 0.25 g/L inhibited deoxyribonucleic acid synthesis by 20% during a 5-h labeling period in 1-wk-old confluent monolayers. Chromium 51 release assay demonstrated that a 22-h exposure to 0.75 g/L of hydrolyzed carrageenan induced the release of 30% of the 51Cr trapped in 1-wk-old confluent monolayers. Scanning electron microscopy showed that the disruption of cellular junctions occurred before cell membrane injury. When we treated the monolayers with drugs commonly used for the treatment of inflammatory bowel disease, including prednisolone, 5-aminosalicylic acid, metronidazole, and 6-mercaptopurine, we were not able to demonstrate a reduction in carrageenan-induced cell injury; however, catalase at 1 mg/ml decreased carrageenan cytotoxicity. These studies demonstrate that hydrolyzed carrageenan produces intestinal epithelial injury in a time- and dose-dependent fashion. Morphologic injury starts at the site of cell junctions and eventually affects cell membrane integrity. Drugs commonly used to treat inflammatory bowel disease do not inhibit carrageenan injury in this cell model system, although catalase does decrease injury.