Clastogenic factors (CF) are diffusible molecules that damage DNA. They are generated within biological media by a variety of physical and chemical stimuli. Their nature and mechanism of action remain largely unknown. Clastogenic activity can be experimentally generated by pulsed ultrasound treatment of human serum. To investigate whether oxygen radicals are involved in the clastogenic activity induced by sonication of human serum, we examined the effects on such clastogenic activity of different oxygen radical scavengers added to human serum before and after sonication. Human serum was sonicated for 50 min at 24 microW/cm2 by pulsed ultrasound. The clastogenic activity of sonicated human serum was examined in the presence or absence of oxygen radical scavengers by measuring the amount of DNA damage induced in autologous human lymphocytes, assessed with the fluorometric analysis of DNA unwinding (FADU). Sonication of human serum generated significant DNA damage in autologous lymphocytes (DNA unwinding averaged 31.79% +/- 2.1 after sonication vs. 12.82% +/- 2.6 in the controls, p < 0.005). Superoxide dismutase (SOD; 500 I.U./ml), catalase (500 I.U./ml), mannitol (50 mM), and glutathione (50 mM) completely prevented DNA damage when added before serum sonication, whereas only mannitol (86%) and glutathione (90%) almost completely inhibited DNA damage when added after sonication. SOD and catalase had only a partial inhibitory effect when added after sonication (49% and 63%, respectively). The prevention of DNA damage was also obtained by an association of subliminal amounts of glutathione (20 mM) and vitamin E (1 I.U./ml). These results suggest that the clastogenic activity generated by sonication of human serum is mediated by oxygen radicals.