Tonsils of human fetuses at the 8th to the 28th gestational week (g.w.) were investigated by electron microscopy, enzyme histochemistry, and immunohistochemistry on cryostat sections. The development of the tonsilla palatina starts during the 14th g.w. when the mesenchyme underlying the mucous membrane of the tonsillar cavity becomes invaded by mononuclear wandering cells. In fetuses of about the 16th g.w. epithelial crypts grow down into the connective tissue and are infiltrated by T-lymphocytes. At the same time, precursors of interdigitating cells (IDC) can be identified among the epithelial cells. Frequently, lymphocytes and IDC-like cells are in close contact. From these findings it is concluded that the infiltrated crypt epithelium in the human tonsilla palatina represents a T-cell region. Primary follicles develop in earlier fetal stages than in all other secondary lymphoid organs. They contain precursors of dendritic reticulum cells and lymphoid cells that belong to the B-cell line. These primary follicles may be considered as the first assemblage of B-cell regions in human fetal lymphoid tissue. The present findings indicate that the formation of different stationary elements during the development of B-cell regions and T-cell regions is an important factor for the homing and antigen-dependent maturation of different subpopulations of immunocompetent lymphoid cells.