The incidence of main nucleolar types in granulocytic precursors was studied in the granulopoietic proliferating compartment (GPC) of patients suffering from chronic phase of the chronic myeloid leukemia (CML) who were treated by the widely used therapy with two different drugs with different mode of action - hydroxyurea (HU) and interferon alpha (IFN-alpha). In comparison with IFN early stages of GPC, i.e. myeloblasts and promyelocytes in patients treated with HU possessed more frequently micronucleoli which are known to reflect the direct as well as indirect inhibition of the nucleolar biosynthetic activities. On the other hand, the incidence of micronucleoli in these cells of a small percentage of patients treated with IFN also reached the average values of these nucleoli which were noted in patients treated with HU.