Chromosomal aberrations (CAs), sister-chromatid exchanges (SCE_s), aneuploidy and proliferative potential (PP) were investigated in peripheral blood lymphocytes of healthy cows (control group-C), BLV-(bovine leucosis virus)-infected cattle without hematological abnormalities (RID - seropositive group (I) and affected with leucaemia (lymphocytosis (LC), lymphoma (L)). Nonrandom chromosomal (marker) aberrations were not found in the cow group at stage LC. The levels of aneuploidy and SCE_s increased in the cow group at stage L compared to the cow group at stage I. Polyploidy: C - 1.9±0.28, I - 3.5±0.22, LC - 6.1±0.82, L - 10.5±0.51 (P < 0.01). Hypoploidy (2n = 58): C - 3.0±0.17, I - 5.4±0.71, LC - 12.1±0.72, L - 14.0±0.65 (P < 0.01). SCEs: C - 3.8±0.26, I - 5.4±0.15, LC - 7.2±0.16, L - 9.7±0.26 (P < 0.01). There are no differences in CAs rates and PP between groups of cows at all the observed stages of leucaemic process. The obtained results are discussed in terms of cytogenetic aspects of leucaemic process in cows.