Lymphocyte cultures were treated with bleomycin to see if individuals who exhibit a high rate of spontaneous chromosome breakage also show an exaggerated response to clastogenic agents. For our experimental group, we used patients with medullary thyroid carcinoma and their immediate family members. Lymphocytes from these individuals tend to show high rates of spontaneous chromosome breakage. The average bleomycin-induced breakage rate in this group was significantly higher than that found in the normal controls. In the control group, the distribution of breakage rates was bimodal, with mean values of 0.68 breaks per cell in one group, and 1.84 breaks/cell in the other. The high rate of induced breakage shown by phenotypically normal subjects may mean that these individuals have a potential sensitivity to mutagen exposure.