Chromosome instability adversely affects animal fertility and reproduction. Analysis of instability can be a valuable diagnostic tool. Helpful tests for assessment of instabilities include the sister chromatid exchange assay, identification of fragile sites, the bleomycin assay and the comet assay. These techniques can be used to assess and compare the chromosome stability of individual breeds of animals. The aim of the study was to assess chromosome stability in boars: Duroc, Duroc x Pietrain and Pietrain x Duroc crossbreds, Polish Large White, and the Neckar, P76 and PIC lines. The study assessed the chromosome stability of boars. The distribution of instabilities in individual breeds was varied. The average frequency of chromatid exchange was 4.8 ± 1.5, while that of fragile sites was 3.9 ± 1.4. The mean level of DNA damage (% tail DNA) was 9.4 ± 8.3, while in the bleomycin assay b/c and %AM were 0.6 ± 0.7 and 44.4 ± 4.1. A higher rate of instability was found in older individuals than in younger ones. The cytogenetic assays used to identify various forms of chromosome instability can be used to evaluate boars intended for breeding.