Immunofluorescent technques were utilized to study the nature and distribution of cells forming the inflammatory infiltrate in Crohn's disease. Tissue from resected ileum (one patient) or colon (one patient) when compared with corresponding control tissues contained increased numbers of B and T lymphocytes. B-cells of IgG class predominated in the mucosal lamina propria, whereas T-cells were prominent in the deeper layers of inflamed bowel. Inflammatory cells bearing Fc receptors were also present in these tissues. Decreased peripheral blood T-cell numbers returned to normal after resection of the involved bowel in the two patients studied. Impaired cellular immunity in Crohn's disease may thus result from T-cell sequestration in or loss from involved segments of intestine.