Cytogenetic damage was investigated in long term lymphoid cell lines derived from normal individuals, patients with Fanconi's anemia (FA), ataxia telangiectasia (AT), xeroderma pigmentosum (XP), and FA heterozygotes. The cell lines were exposed to various concentrations of four chemical clastogens, including the alkylating agents diepoxybutane, mitomycin C, and nitrogen mustard, as well as the antitumor glycopeptide bleomycin. The FA cells exhibited chromosomal hypersensitivity to all four clastogens and could be distinguished from the other genotypes. AT cells were identified by bleomycin, while FA heterozygotes could not be reliably detected. Discriminant function analysis was used to describe the cytogenetic response to the various clastogens. This method might prove useful for evaluation and classification of multivariate chromosome breakage studies.