Investigations with the electron microscope on peripheral blood of patients with Hodgkin’s disease revealed parallel tubular structures in the cytoplasm of the lymphocytes, similar to those described in the literature. In normal subjects 2–9% of the lymphocytes were found to contain these structures. The number of such lymphocytes is increased in most patients as compared to normals. A correlation between an increase of inclusions containing lymphocytes and the histological or clinical classifications could not be established. An elevation of lymphocytes with tubular structures could be found in both favourable and unfavourable histological and clinical subgroups of Hodgkin patients, but especially in all groups with an unfavourable prognosis. The origin and function of the parallel tubular structures is still unknown, but their increase in lymphocytes of patients with Hodgkin’s disease is of some interest in view of the involvement of these cells.

Summary. The investigation of cells larger than normal in the peripheral blood of patients with Hodgkin's disease by electron microscopy revealed, besides cells of the granulocytic series and parts of megakaryocytes, four other categories. The cells of the mononuclear phagocytic system could easily be recognized. The so‐called immunoblasts seen in light microscopy as dark basophilic lymphoid cells apparently consist of cells from B‐ as well as T‐cell origin, according to their ultrastructural characteristics. All these cells could also be found in nine healthy control persons, two patients with cytomegalovirus disease and two with reticulosarcoma. The fourth category was formed by a rare cell, only encountered in patients with disseminated Hodgkin's disease. They probably represent the moderately basophilic blast cell seen in light microscopy, considered to indicate dissemination of Hodgkin's disease. Submicroscopic details of these cells are given. It is very improbable that they belong to the monocytes or to the immunoglobulin producing cells. A relation to the thymus derived immunoblasts could not be excluded, while on the other hand the morphological features closely resembled those of cells found in cases of type I and type III of reticulosarcoma (Mori & Lennert, 1969). Thus, in this investigation the number of possibilities for the origin of the typical cells indicating splenic involvement of Hodgkin's disease was limited, but a definite conclusion could not be reached on the basis of the presently available morphological data.