To investigate the relation of immunoglobulin-containing cells in the colonic mucosa to mucosal inflammation, we immunohistochemically examined the localization of immunoglobulin-containing mononuclear cells in the lamina propria in dextran sulfate sodium induced colitis in mice. Mice were treated repeatedly with 3% dextran sulfate sodium (MW 54,000) solution or distilled water for a total of 170 days (chronic model), or for 85 days (subacute model) or for 10 days (acute model). IgG, IgA, and IgM-containing mononuclear cells were studied by enzyme immunostaining. The number of IgA- and IgG-containing cells gradually and significantly increased in the acute, subacute, and chronic models, in that order (P < 0.01 or 0.05). However, the numbers of IgM-containing cells in the three models were similar to that in the controls. These findings resembled those of human ulcerative colitis. In this dextran sulfate sodium-induced colitis, IgA-containing mononuclear cells may play an essential role in the mucosal immune system is the acute, subacute, and chronic phases. The finding that IgG-containing mononuclear cells increased substantially in the chronic phase suggests that IgG plays an important role in the mucosal inflammatory reaction during the chronic phase.