Peripheral blood lymphocytes from eight Fanconi anemia (FA) patients, 14 FA heterozygotes, and nine normal subjects have been tested for their susceptibility to chromosomal breakage induction by diepoxybutane (DEB) and by two peroxides. In addition, the effect of five antioxidants was investigated in standard cultures and in cultures stressed either with DEB or with butylhydroperoxide (BHP) or with hydrogen peroxide (H2O2). DEB, BHP, and H2O2 dramatically increased the chromosomal breakage levels in homozygous and heterozygous FA cells. A partial correction of chromosomal instability was obtained by treating the patients' lymphocytes with antioxidants. A "protective" effect was also noted in the DEB or peroxide-stressed lymphocytes of patients and heterozygotes, grown in the presence of antioxidants.