The investigation of nucleoli is a simple but useful complementary approach for the evaluation of individual leukemic cells in addition to other hematological laboratory procedures. Since main nucleolar types represent the morphological expression of nucleolar biosynthetic activities, i.e. RNA transcription, the incidence of functionally dominant nucleolar types in leukemic cells reflects the state of these cells which is related to that transcription. Similarly, the investigation of AgNORs in individual leukemic cells may provide a complementary useful information on their proliferation activity.