The production of an extracellular layer of polysaccharide, termed the capsule, is a common feature of many bacteria. Capsules play a vital role in permitting evasion of the host immune specific and nonspecific defenses as well as helping in adhesion for colonization of host tissue, Colanic acid capsule is usually produced in low quantities and is a common feature of several enteric bacteria. The role of the colanic acid capsule in aiding adhesion and virulence was investigated. Encapsulated and unencapsulated cells were injected into granuloma pouches that were formed on the backs of rats. During the first 50 h, the viability of the matched encapsulated and unencapsulated cells decreased. These studies showed that colanic acid capsule does not confer resistance to the bactericidal activity of serum or to phagocytosis in vivo, since there was no significant difference in the survival rates of both strains over time. Adherence studies were conducted in monolayers of human carcinoma intestinal cells (T84) with the same matched strains. After incubating radioactively labeled bacteria with the colon cells, the level of adherence was determined by measuring the radioactivity remaining in the tissue culture wells. The results of these experiments indicated that the unencapsulated cells adhered more readily to the intestinal cells, suggesting that colanic acid capsule interferes with adherence in this model system.