Three points related to the subject of chromosome-aberration tests and carcinogenicity screening are dealt with in this paper. First, the relation between chromosome rearrangements and oncogenesis is briefly discussed, to stress the relevance of cytogenetic assays to assessment of the carcinogenic potential of drugs. Secondly, some recent ideas concerning the origin of chemically-induced chromosome aberrations are reported. In particular, the involvement of specific DNA lesions caused by alkylating agents and the possible mechanism of action of tumour promoters with regard to the induction of chromosomal breakage are recalled. Finally, a number of in vivo and in vitro mammalian cytogenetic assays are briefly described.