The submicroscopic morphology of blood and bone marrow elements in a patient with plasma ceil leukemiaBence Jones proteinuria (type K) and hypogamma-globu- linemia is described. Highly undifferentiated cells were rarely seen and tentatively called blast cells. Two other cell types Plasma cell leukemia with a more complex ultrastructural organization ressemble the ‘intermediate’ lymphocytes and the plasmacytic elements and probably represent different functional stages of the same cell. In particular the absence of large polyribosomes and of dilated ergastoplasmic cisternae, and the presence of small polyribosome rosettes formed by 5–10 ribosomes, are correlated with the hypogammaglobulinemia and with the high L chain synthesis in this p