Mucosal samples from 16 patients with idiopathic inflammatory bowel disease were examined immunohistologically using several monoclonal antibody combinations, and the results were compared with those obtained in other colonic inflammatory disorders and in normal mucosa. Within and around lymphoid follicles, most T cells expressed the restricted common leucocyte antigen (CD45R, displayed by unprimed T cells). Conversely, most lamina propria T cells were negative for CD45R but stained positively with UCHL1 (a monoclonal antibody recognizing an antigen displayed by primed T lymphocytes). The proportions of T-lymphocyte subpopulations in normal and inflamed mucosa were similar, except that CD6-negative, CD7-positive cells were significantly more frequent in inflammatory bowel disease. A characteristic feature of ulcerative colitis and Crohn's colitis was marked infiltration by CD45R+ lymphoid cells that did not coexpress T- or B-cell surface antigens; some stained positively with plasma cell reagents, suggesting that they may be activated B cells. The observations in the colitis sections are consistent with the interpretation that T and B cells alter their surface antigen expression as they emerge from follicles and enter the inflamed lamina propria.