Lymphocytes within fetal human ileum were studied by immunocytochemistry to determine the appearance of T cells in human small intestine and the role of enteric antigen in the accumulation of cells of the suppressor/cytotoxic phenotype in the gut epithelium. In fetal human gut epithelium, cells bearing the pan T cell marker UCHT1 (CD3) were present in all of the specimens studied (11-19 weeks gestation). Of these, UCHT4+(CD8, suppressor/cytotoxic phenotype) predominated over the leu3a+(CD4, helper/inducer phenotype), although the differences were not as marked as in postnatal gut. UCHT1+ cells were also present in the lamina propria, frequently as small aggregates beneath the epithelium.