The oxygen-enhancement ratio (o.e.r.) for 3H- and 125I-induced cell death at 4 degrees C was determined in cultured Chinese hamster cells. The o.e.r. for cell death induced by 3H-thymidine was 3-2, essentially the same value as that previously reported for X-ray induced cell death. For cell death induced by 125I-iododeoxyuridine (125IdUrd), the o.e.r. was less than 1-4. The lower o.e.r. for 125I-induced death was not due to the presence of the base analogue itself, since cells that had incorporated unlabelled IdUrd and were X-irradiated had an o.e.r. of 2-8 and cells that were inactivated by 3H-IdUrd damage at 4 degrees C had an o.e.r. even greater than 3. These results suggest that 125I-decay damage, like high-linear-energy-transfer radiation damage, is only slightly increased by the presence of oxygen.