Peripheral blood samples and skin fibroblast cultures from five children with Fanconi’s anemia and four normal children were irradiated with 10 and 100 rads. Nonirradiated duplicate cultures were used as controls. Blood cultures were harvested after 52 and 72 hr of incubation and fibroblast cultures were harvested 24 hr after irradiation. The frequency of chromosomal breakage caused by irradiation was calculated by subtracting figures for nonirradiated samples from those for irradiated samples. After irradiation, the number of breaks/cell/rad in samples from Fanconi’s anemia was approximately four-fold greater in lymphocytes and twofold greater in fibroblasts than in the controls. The number of dicentrics and rings per cell in cultured lymphocytes after 100 rads was significantly greater in the Fanconi’s anemia samples than controls. Therefore, in vitro, chromosomes of cells from children with Fanconi’s anemia were significantly more radiosensitive than those of controls (p < 0.01).