A sensitive reverse haemolytic plaque assay to detect lymphokine-secreting T cells, and Northern blot analysis to detect expression of lymphokine messenger RNA (mRNA) were used to study interferon-gamma (IFN-gamma) and interleukin-2 (IL-2) production in the mucosa of children with Crohn's disease or ulcerative colitis (UC), and in histologically normal mucosa from patients without inflammatory bowel disease. In the mucosa of most patients with UC and control patients, IL-2- and IFN-gamma-secreting cells were absent or were present at only low levels. In contrast, in mucosa from patients with Crohn's disease, lymphokine-secreting cells were readily detectable (3-18%). IFN-gamma mRNA was detected by Northern blot analysis in 5/6 Crohn's tissues, but only in 1/5 UC samples and none of nine samples of control mucosa. These studies reveal an ongoing cell-mediated immune response in the mucosa in Crohn's disease.