Sensitivity of phytohaemagglutinin-stimulated lymphocytes to bleomycin clastogenicity is increased in patients with tumours in organs and tissues that are in direct contact with the external environment, such as the mucosa of the head and neck. Sensitivity to bleomycin may reflect a genetically determined hypersensitivity to certain genotoxicants and therefore may be important in the carcinogenic process. In this study the applicability of bleomycin genotoxicity was investigated in cultured lymphocytes of forty individuals without a tumour history. No correlations were observed with increasing age or the well-known head and neck cancer risk factors alcohol and tobacco consumption. Since inter-individual variation in sensitivity greatly exceeded intra-individual variation, our results suggest that an elevated bleomycin clastogenicity score may identify individuals who have a constitutional hypersensitivity towards certain genotoxicants and may show an increased cancer susceptibility.