Structural chromosome damage, sister chromatid exchange (SCE), and proliferation kinetics were studied on lymphocyte cultures from the peripheral blood of two sibs exhibiting signs of Fanconi anemia, their relatives, and control individuals. While the rate of spontaneous chromosome breakage was at the lower limit of that known for Fanconi anemia in our patients, a distinctly greater increase than in controls of breakage frequency could be induced by isoniazid (INH), 4-nitroquinoline-1-oxide (NQO), and diepoxybutane (DEB) in their lymphocytes. Increased aberration frequencies as compared with controls were also observed in the clastogen-exposed lymphocyte cultures of the parents of both sibs, but in some experiments (NQO, DEB 24 h) only in the cells of the healthy brother. There was an increase in the breakage rate of bromodeoxyuridine (BrdU)-labeled consecutive mitoses under the action of NQO, but a decrease with INH as the test clastogen. No significantly higher SCE frequency was found throughout the study in untreated and clastogen-exposed FA lymphocytes as compared with the respective controls. Proliferation was clearly inhibited by INH and NQO as indicated by a distinct increase of the percentage of BrdU-labeled first and a drastic decrease of third metaphases. The present test clastogens were shown not only to be suitable for ensuring the diagnosis of FA in patients with a low incidence of spontaneous breakage but also for determining clastogen-sensitive heterozygotes. According to these results cross-link repair cannot be the only mechanism affected by the basic defect of Fanconi anemia.