This paper describes the histological picture of four tumours of the follicular compartment of the lymph node, in which the proliferating cell appeared to be the dendritic reticulum cell (DRC). This assumption was based on the results of light microscopical, ultrastructural, immunological, and enzymehistochemical investigations. The tumour cells resembled DRC's closely in (1) the striking pattern of interdigitations and occasional tight junction-like contacts between the neoplastic cells on electron microscopical analysis; (2) presence of receptors for the activated third component of complement on the membrane of the cells; (3) absence of monoclonal immunoglobulins and T-cell antigen on the surface and of lysozyme, alpha 1-antitrypsin or alpha 1-antichymotrypsin in the cytoplasm of the neoplastic cells. Moreover, (4) the tumour cells showed moderate alpha-naphtyl acetate esterase, weak to absent acid phosphatase and (with one exception) strong 5-nucleotidase activity. Furthermore, (5) the neoplastic cells expressed Ia-like antigens on the surface in all four cases. The relation with follicle centre cell lymphomas, the differential diagnosis and clinical data are discussed.