The ultrastructure of hypertrophic nucleoli in large cells from each of 5 successive cases of non-Hodgkin’s lymphoma (regardless of predominant cell type) showed irregularly rounded or ovoid, mostly compact forms with few scattered light spaces. Occasional nucleolonemal forms were noted. Few ring forms or transitional forms between ring and compact or nucleolonemal types were present. Granular components occupied 78.5 ± 1.0% of the mean total area of 132 lymphoma large cell nucleoli of all configurations studied. Hypertrophic nucleoli in 27 phytohemagglutinin (PHA)-stimulated lymphocytes were predominantly nucleolonemal with 74.2 ± 10.6% granular composition. The qualitative and quantitative distribution of nucleolar granular moieties did not vary significantly between individual cases of lymphoma nor quantitatively between these and nucleoli in PHA-stimulated lymphocytes. These observations lend further credence to the suggestion of the lymphocyte as a common progenitor cell type for the large B and T cell of malignant lymphomas. Some functional similarity of their nucleoli and those of PHA-stimulated lymphocytes in their participation in ribonucleic acid synthesis is implied. Generally compact nucleoli of lymphomas in contrast to nucleolonemal forms of PHA-stimulated lymphocytes may be pathognomonic for large lymphocytic cells undergoing neoplasia.