Peripheral blood lymphocytes of patients with Fanconi's anemia (FA) were tested for their susceptibility to chromosome breakage by caffeine, chloramphenicol, actinomycin D, methylmethanesulfonate, nitrogen mustard, mitomycin C, decarbamoyl mitomycin C, N-methyl-N′-nitro-N-nitrosoguanidine, 4-nitroquinoline 1-oxide, 8-methoxypsoralen, and ⁶⁰Co γ-ray and ultiviolet irradiations. A definitely abnormal response of the chromosomes of the FA cells was found when they were treated with nitrogen mustard and mitomycin C and after the irradiation with long-wavelength ultraviolet light in the presence of 8-methoxypsoralen. The specifically increased susceptibility to these compounds, which can introduce interstrand cross-links into DNA, is interpreted as an indication that the FA cells are defective in the repair mechanism to tolerate the cross-links produced in their DNA. The impairment of the capacity to tolerate the lesions produced in DNA may be implicated in the increased risk to develop malignant neoplasms in this hereditary disorder.