The level of DNA single strand breaks, chromosomal abnormalities and sister chromatid exchanges and the possibility of its normalization with oligopeptide bioregulator Livagen and cobalt ions in the lymphocyte culture from patients with breast cancer have been studied. The results show that the genome of ductal breast cancer patients is characterized by the high density of DNA single strand breaks, high frequency of chromosomal abnormalities and increased levels of chromatin condensation. The usage of Livagen and cobalt in the form of modifying agents has a protective effect by all studied parameters. The obtained results allow us to conclude that research of lymphocytes of ductal breast cancer patients using the analysis conducted by us, can be useful in assessing the therapeutic effect in the treatment of breast cancer patients.