Exposure to ionizing radiation has long been suspected to increase mutation load in humans. Nevertheless, such events as atomic bombing seem not to have yielded significant genetic defects. The Chernobyl accident created a different, long-term exposure to radiation. Clean-up teams (or 'liquidators') of the Chernobyl reactor are among those who received the highest doses, presumably in some combination of acute and chronic forms. In this study, children born to liquidator families (currently either in the Ukraine or Israel) conceived after (CA) parental exposure to radiation were screened for the appearance of new fragments using multi-site DNA fingerprinting. Their sibs conceived before (CB) exposure served as critical internal controls, in addition to external controls (non-exposed families). An unexpectedly high (sevenfold) increase in the number of new bands in CA individuals compared with the level seen in controls was recorded. A strong tendency for the number of new bands to decrease with elapsed time between exposure and offspring conception was established for the Ukrainian families. These results indicate that low doses of radiation can induce multiple changes in human germline DNA.