Thin sections from nodes and bone marrow of patients with malignant blood diseases have been investigated with the electron microscope. "Granular bodies" and "vesicular bodies" are found in the cytoplasm of Sternberg cells, and "vesicular bodies" in leukemic cells. An organized ergastoplasm is only to be seen in malignant plasmocytes from myeloma, in isolated plasmocytoma and in lympoid cells from macroglobulinemia. Its development and its relation to protein production and secretion are demonstrated. The submicroscopic organization of cells is described and commented upon.