The ultrastructure of the cells of six B lymphocyte hybridomas producing monoclonal antibodies does not, in principle, differ from the mouse myeloma cells. Their cytoplasm contains mostly membrane bound polyribosomes, Golgi apparatus and numerous membrane structures. The dominant finding, however, was the presence of a great number of A particles which occur almost exclusively in the cisterns of the endoplasmatic reticulum. A few particles were found in the mouse thymoma. C-virus particles were detected to a lesser extent in hybridoma and myeloma cells. The finding of the C-type particles in the hybridomas that produce monoclonal antibodies is very important; monoclonal antibodies of the mouse origin have recently become a powerful tool not only in the diagnosis, but also in the therapy of human diseases.