It has been shown previously that sensitivity to the induction of chromosome damage by ionizing radiation is, on average, higher in G2 or G0 lymphocytes of breast cancer patients than of normal healthy controls. The authors suggested that elevated chromosomal radiosensitivity may be a marker for breast cancer predisposition. To investigate whether the G0 micronucleus assay is a true surrogate for the more demanding G2 metaphase assay, both tests have now been performed on the same blood samples from 80 patients. For the G0 micronucleus assay, cells were exposed to 3.5 Gy 137Cs gamma-rays 6 h before mitogenic stimulation, treated with cytochalasin B at 24 h post-stimulation and harvested at 90 h. For the G2 assay, at 72 h after stimulation cells were given 0.5 Gy X-rays and harvested 90 min later. Previous observations were confirmed, now with much larger numbers of donors, in that approximately 40% of breast cancer patients showed elevated sensitivity in the G2 assay (135 patients, 105 normals) and 25% in the G0 assay (130 patients, 68 normals). However, there was no correlation between G2 and G0 sensitivity for the 80 patients tested (r = -0.001, p = 0.99). Most of the sensitive patients were either G2 or G0 sensitive, with only 4% sensitive in both assays. The results suggest that different mechanisms of chromosomal radiosensitivity operate in G2 and G0 cells and that, in general, each chromosomally radiosensitive patient is defective in only one such mechanism, possibly via mutation (or polymorphism) of a single gene. Such mutations may confer cancer predisposition, of low penetrance, in a substantial proportion of patients.