Two opposite processes such as apoptosis and cell division cycle play an important role in cancer biology. During apoptosis cells die by degrading proteins and genome, whereas in the cell cycle cells divide and the genome is duplicated. There are very few studies on the role of cell cycle proteins in apoptosis although their role is distinctly different from that in the cell cycle. The significance of expression of cyclin A and other cyclin cell proteins (eg. Cdk2) in apoptosis remains to be investigated. The aim of this study was to characterize the distribution pattern of cyclin A by using the stereptavidin - biotin - peroxidase technique in K-562 cells treated with doxorubicin. The analysis of cell cycle phases using cytophotometric methods to estimate the cellular response to doxorubicin was also used. Studied cells were treated with doxorubicin in the range 0.5; 5.0 and 10 microM. Expression of cyclin A in K-562 was 32.2; 41.8; 69.9%, respectively, according to doses of doxorubicin. The number of apoptotic cells was increasing together with the increase of doxorubicin doses as well as positive labeling for cyclin A. After doxorubicin treatment decrease of G1/G0 phase and the growth of cell percentage with dose dependent manner at G2/M phase, compared to control was observed. The results allow to suggest that expression of cyclin A may have pro-apoptotic role however more studies are required to clarify whether and what role cyclin A plays in apoptosis.