Recent studies indicate that intestinal lamina propria T cells are highly special ized lymphocytes, which differ from T cells in other compartments of the immune system in several respects. In the present study phenotypic and functional characteristics of lamina propria T cells and their possible relation to mucosal growth will be discussed. Lymphocytes from human and nonhuman primate intestine were isolated by an enzymatic procedure. Lymphocytes were studied using dual-color immunofluorescence (FACS) and functional in vitro assays. CD4 positive (helper-) lamina propria T-cells lack the CD45RA antigen and express the CD45RO antigen. This phenotype is characteristic for memory T cells. In addi tion intestinal T cells express IL-2 receptors and IL-2 receptor mRNA. and are able to synthesize high amounts of IL-2. Functional studies in nonhuman primates infected rectally with Chlamydia trachomatis have shown that lamina propria T cells do not proliferate after stimulation with antigen but rather provide helper function for immunoglobulin synthesis. The intestinal lamina propria therefore contains highly specialized T cells which have the phenotype of memory T cells and which are activated. Functionally these T cells can be characterized as differentiated effector lymphocytes. Recent studies from other laboratories have shown that the pattern of lymphokines produced by lamina propria T cells and the responsiveness to certain lymphokines also differ from those of other lymphocyte popula tions. Since T-cell-derived lymphokines are also important regulators for epithelial growth and differentiation as well as for connective tissue metabolism, lamina propria T cells might be of major importance in mucosal growth and transformation.