Mean diameter of nucleolar bodies (nucleoli without the perinucleolar chromatin) per cell was studied in human leukemic myeloblasts represented by K 562 and Kasumi 1 cell lines which originated from chronic and acute myeloid leukaemia. The measurement of mean diameter of nucleolar bodies in specimens stained for RNA was very simple. Such approach eliminated the variability of the perinucleolar chromatin discontinuous shell which might influence the measured nucleolar size as suggested by earlier studies. Ageing of K 562 myeloblasts produced a significant decrease of cells in S+G2 phase of the cell cycle accompanied by a significant reduction of mean diameter of nucleolar bodies (MDNoBs) per cell. In contrast, treatment of Kasumi 1 myeloblasts with histone deacetylase inhibitor - Trichostatin A - produced a large incidence of resistant cells in S+G2 phase which were characterised by a large increase of MDNoBs. Thus, MDNoBs in leukemic myeloblasts might be a helpful tool to estimate the incidence of cells in the S+G2 phase at the single cell level in smear preparations when the number of cells is very small.