The detection of chromosomal damage serves as a tool for the verification of the genotoxic effects of chemical substances in vitro. We used conventional cytogenetic analysis in order to test for the potential genotoxic action of the insecticide thiacloprid (the active ingredient in commercial preparation CALYPSO 480 SC). The test cultures of bovine lymphocytes obtained from the peripheral blood were incubated with the insecticide in concentrations of: 30, 120, 240 and 480 μg.ml−1 for 24 and 48 hours. After 24 hours of incubation, we observed that the increasing concentrations resulted in a significant (P < 0.05; P < 0.01) increase in the frequency of DNA damage. Our experiments showed the presence of aberrations of a non-stable type (chromatid and chromosome breakage). The conventional chromosome analysis was supplemented with fluorescence in situ hybridization for the detection of numeric and stable structural aberrations. Whole chromosome probes for bovine chromosomes 1, 5 and 7 (BTA 1, BTA 5 and BTA 7) were used in the experiments.