The surface of the intestinal mucosa is under constant exposure to numerous antigens from microorganisms and dietary products. Gut-associated lymphoid tissue together with the nonspecific harriers like digestive proteases, intestinal mobility, the commensal microflora and mucous coat are believed to provide protection of the host. Moreover, intestinal plasma cells produce large amounts of IgA which can traverse the mucosal membrane and may to some extent prevent entry of foreign antigens. The majority of antigens are believed to be presented to B cells and T cells by antigen-presenting cells. T cells are then activated to proliferate and produce lymphokines that are a prerequisite for B cell activation, differentiation and immunoglobulin production. The mucosal areas of the intestine are populated by a large number of T lymphocytes. However, little is known about their functional repertoire and their immunoregulatory properties. Employing monoclonal antibodies (mAbs) which are directed at human T cell differentiation antigens, it was recently demonstrated that both CD4 § and CD8+ T cells exist in lamina propria with a predominance of the CD4 subset [1-3]. In addition, the majority of lamina propria T lymphocytes (LPL-T) appear to employ an ctJ3 T cell receptor type for antigen recognition whereas only few cells express a 75 receptor [3]. In contrast to peripheral blood, however, an enhanced fraction of T cells are reactive with mAbs specific for the interleukin-2 (IL-2) receptor (CD25) and HLA-DR, respectively [2-4], and express the CD45RO isoform [5].