The distribution of mononuclear cell subsets has been studied in human adenoids, tonsils and peripheral blood (PB) by evaluating the presence of surface immunoglobulins, E-rosette formation, receptors for IgG Fc and for complement, alpha-naphthyl acetate esterase (ANAE) cytochemistry, reactivity with peanut lectin (PNA) and with monoclonal antibodies (McAb) (OK panel). Adenoids and tonsils, compared to PB, contain (1) fewer macrophages and T cells but more B cells; (2) higher proportions of ANAE negative, complement receptors and Ia-like antigens bearing T lymphocytes; (3) higher percentages of cells reacting with the McAbs OKT9 and OKT10 ("immature" lymphoid cells). In both adenoids and tonsils, clusters, formed by a central heavily ANAE stained interdigitating cell surrounded by lymphocytes with a sickle-shaped ANAE reaction, were found. Analogous clusters have been previously described in mice and human thymus. Two major hypotheses could be put forward: (1) adenoids and tonsils contain "immature" lymphoid cells undergoing education process, or (2) the above organs contain lymphocytes activated by a constant exposure to bacterial antigens or mitogens.