The purpose of this investigation is to define the T lymphocyte subpopulations and function of peripheral blood and intestinal lamina propria lymphocytes in patients with Crohn's disease, an idiopathic intestinal inflammatory disease which is thought to be immunologically mediated. Lymphocytes were obtained from peripheral blood or isolated by an enzymatic digestion method from surgically resected intestinal specimens. Lymphocytes were then examined by dual immunofluorescence using a dual laser flow cytometer, using combinations of monoclonal antibodies which have been shown to correlate with different lymphocyte functions. In peripheral blood, patients with Crohn's disease have a similar proportion of lymphocytes in the suppressor-inducer subset, in the suppressor-effector subset, and in the cytoloytic T cell subset compared to controls. Interestingly, patients with Crohn's disease and controls had a significantly decreased proportion of lymphocytes having the phenotype of suppressor-inducer cells and the phenotype of suppressor effector cells. These results indicate that the intestinal lamina propria contains distinctly different subpopulations of T cells compared to peripheral blood which are presumably involved in mucosal host defense mechanisms. Further definition of the function of intesitnal lymphocytes will provide greater under standing of the mechanisms involved in intestinal inflammation in inflammatory bowel disease.