Previous experiments with the carrageenan model for ulcerative colitis demonstrated that the inflammatory response in guinea pigs can be enhanced by immunization with Bacteroides vulgatus and subsequent feeding of this organism to experimental animals. The studies reported here show that antigens extractable from the bacterial outer membrane by EDTA are responsible for this effect. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to analyze the outer membrane proteins from various strains as well as the lipopolysaccharides (LPS) extractable by the phenol-water method. Although the observed pattern of outer membrane proteins was complex, the strains could be divided into two electrophoretic types (phenons) on the basis of immunoblotting against a panel of antisera. Cross-absorbed antisera used in immunoblotting experiments identified four outer membrane proteins uniquely associated with the phenon capable of enhancing the colitis inflammatory response. These proteins had molecular weights of 100,000, 57,000, 34,000, and 21,000 when measured in 8% to 12% acrylamide gradient sodium dodecyl sulfate gels. Other antigens identified included at least one type of smooth LPS, three types of rough LPS, and a common antigen of 30,000 molecular weight among the strains of B. vulgatus tested. The outer membrane preparations were used in animal immunization and challenge experiments, and the severity of colitis was correlated with one electrophoresis type. The potential role of membrane proteins in the enhancement of colitis is discussed.