Abstract.  The signal transducers and activators of transcription – STAT5A and STAT5B – take part in the regulation of many essential physiopathological processes. They influence the cell cycle, apoptosis and the proliferation of different types of cell lines. The STAT5 proteins are induced in response to multiple haematopoietic cytokines. Because they are constitutively active in certain haemato-oncologic diseases, it is also suggested that they play an important role in leukaemogenesis. However, function of these proteins in haematopoietic cell transformation and proliferation is not clear. The aim of this study was to evaluate the impact of perturbation of STAT5 expression [using oligodeoxynucleotide (ODN) against STAT5 mRNA], on the clonogenicity and survival of selected human leukaemic cell lines, HEL, HL-60, K562, TF-1. We analysed the effect of ODN pre-treatment on the cell clonogenicity in methylcellulose cultures according to the time and the temperature of exposure. Moreover, we attempted to estimate apoptosis induced in examined cells, by flow cytometry using combined Annexin V-PI staining and the TUNEL method. We also applied the RT-PCR method to analyse Bax and Bcl-xL gene expression. We found that the perturbation of STAT5 expression with antisense oligonucleotides caused a decrease in the proliferative potential of human K562 and TF-1 cell lines. Also, we observed higher induction of apoptotic cell death in the K562 and TF-1 cells incubated with the antisense STAT5A ODNs. We did not notice any impact of ODNs on the HL-60 and HEL cells. Our studies using STAT5 antisense oligonucleotides showed that these proteins may be critical in the regulation of growth and apoptosis of some types of leukaemic blasts.