Individual sensitivity to mutagens has been considered to play an important role in head-and-neck squamous cells carcinoma (HNSCC) development. The bleomycin test was introduced for establishing constitutional susceptibility to mutagens (T.C. Hsu, D.A. Johnston, L.M. Cherry, D. Ramkisson, S.P. Schantz, J.M. Jessup, R.J. Winn, L. Shirley, C. Furlong, Sensitivity to genotoxic effects of bleomycin in humans: possible relationship to environmental carcinogenesis, Int. J. Cancer 43 (1989) 403-409). Its criteria are based on scoring of chromosome aberrations (CAs, mainly breaks) in Giemsa-stained chromosomes. Fluorescence in situ hybridization (FISH) offers an easy method for analysis of translocations, acentric fragments and dicentrics. In the present study FISH was applied in the analysis of bleomycin-induced CAs of the HNSCC patients and controls. The results proved that FISH is a complementary method to the classical staining in monitoring of bleomycin-induced CAs.