Immunohistochemical analysis of immunocompetent cells in the colonic mucosa was performed with carrageenan-induced experimental colitis in rabbits. Colitis was induced by seven months of oral administration of lambda-degraded carrageenan following immunization with the same substances containing Freund's complete adjuvant. In the colonic mucosa with colitis, IgG- and IgM-containing cells were significantly increased in number (IgG: 540 +/- 94/mm2 in experimental group, vs. 120 +/- 54/mm2 in control, P less than .05, IgM: 55.0 +/- 19.7/mm2 in experimental group, vs. 6.7 +/- 2.4/mm2 in control, P less than .05). There was no significant increase of IgA-containing cells either in number or in proportion to the total mononuclear cells. These changes, induced by carrageenan in rabbits, had resembled those in human ulcerative colitis well. These observations suggested an impairment of the IgA-regulated local immune system and an abnormality in the differentiation process of immunoglobulin-secreting cells.