Immunohistochemistry with monoclonal antibodies to the T cell receptor V beta regions 5, 6, 8 and 12 was used to determine whether normal intestinal lymphocytes that are potentially exposed to many bacterially derived superantigens show any preferential expression of particular V beta regions compared with the blood. No difference between V beta expression in the mucosa and the blood was observed, suggesting that they share a common pool of alpha beta T cells and that there is no expansion of alpha beta T cells in response to bacterial "superantigens" in the gut. The T cell receptor V beta expressed by the activated T cells in the lamina propria of bowel from patients with Crohn's disease was also studied. There was no increase in V beta 8 expression in these cells, suggesting that the increase in V beta 8 observed in the blood and mesenteric nodes of patients with Crohn's disease is not of primary importance in the aetiology of the disease. Finally, V beta expression by mucosal T cells in coeliac disease was studied. There was no difference in V beta use by T cells in coeliac disease and those in the blood and normal jejunum.