Nucleolar RNA optical density (concentration) measurements at the single cell level indicated that differentiation of lymphocytes is accompanied by a slightly decreased nucleolar RNA concentration in contrast to the cytoplasmic rim around the nucleus. On the other hand, the nucleolar size was markedly reduced and the cytoplasmic rim surrounding the nucleus was reduced only weakly. Concerning the calculated rough estimate of the RNA content, the differentiation induced its larger decrease in the nucleoli than in the cytoplasmic rim. These observations indicated that the nucleolar RNA concentration and RNA content together with the nucleolar morphology are more sensitive markers of the differentiation process than the RNA concentration and content in the cytoplasm. Thus, the nucleolar RNA transfer to the cytoplasm in advanced differentiation steps might still be going on regardless of the decreasing or inhibited nucleolar biosynthetic activity. In addition, the presence of ring-shaped nucleoli and micronucleoli characteristic of mature and terminal lymphocytes in some lymphocytic less differentiated steps, i.e., lymphoblasts and prolymphocytes, might indicate the premature differentiation state of such cells.