The intestinal lymphoid compartment has a rather stable composition throughout life. However, both during early neonatal development and at high age unique cell populations can be recognized at distinct sites in the intestinal tissue. Directly after birth all intestinal CD3+cells are found in the lamina propria. At this time the epithelium does not contain T cells. These CD3+lamina propria lymphocytes co-express both TCRβand TCRδchains, probably reflecting the expression of a TCRβδheterodimer on the cell surface. Cells with this particular phenotype are present in comparable numbers in the lamina propria of both neonatal euthymic and athymic mice, indicating the thymus-independent nature of these cells. During aging the frequency of TCRαβ+CD8αα+intestinal T cells increases. These cells are also considered to be thymus-independent. The appearance of high numbers of CD4+CD8αα+intestinal T cells in aged mice is even more striking. It is postulated that the neonatal TCRβδ+cells, and probably also the CD4+CD8αα+cells as found in old mice, are intermediates in the extrathymic differentiation pathway of TCRαβ+CD8αα+intestinal T cells.