Nine dairy cows aged four to eleven years were subjected to examination by chromosomal analysis. These cows had been found, by haematological examination, to suffer from leucosis. The study also covered one ten-day-old calf - heifer. The test group included two dam-daughter pairs. The animals belonged to the Black-Pied Lowland breed. The blood was sampled from vena jugularis and the karyotypes were processed and evaluated by the method after Moorhead et al. (1960), modified by Lojda et al. (1974). A list was kept for each animal. The tested animals were included in classes by the percentages of the chromosome aberrations: class I - two animals (up to 10% of aberrations), class II - seven animals (from 10% to 20% of aberrations), class III - one animal (above 20% of aberrations). Hyposomy was found in all cases, polysomy and hyperploidy in four cases. Structural aberrations were observed in nine cases, breaks being the most frequent anomalies (7 cases). Breaks on sexual chromosomes were observed in five cases, including the dam-daughter pairs; centric fusion occurred in one case and mixed aberrations in two cases.