Lymphoid tissue in formalin fixed and snap frozen human fetal ileum has been studied using immunohistochemistry. At 11 weeks gestation clusters of cells expressing CD4 (leu-3a positive) are present in fetal ileum but these do not express CD3 (UCHT1 negative) and are probably macrophages. Aggregates of lymphoid tissue are apparent from 14 weeks gestation which contain T cells of helper/inducer and suppressor/cytotoxic phenotype. Both B and T cells are present at 16 weeks but with no cellular zonation. By 19 weeks, distinct follicles of B cells are present surrounded by T cells of helper/inducer and suppressor/cytotoxic phenotype. Follicular dendritic cells are also present within the B cell areas. The B cells at this age express surface IgM and IgD, C3b-and C3d-receptors. They also express the antigen CD5 which has been shown by others to be present on some fetal B cells but which is almost exclusively associated with T cells in the adult. HLA-D region antigens are present on apparently all of the cells within the fetal lymphoid follicles. The antigen on activated B cells, CD23 (recognized by MHM6), was present on some cells scattered within the B cell follicle. This is indicative of antigen independent B cell proliferation.